Girlfriend's guy friend

So we started out as all friends and co-workers at the begging of the summer. This other guy is 21 and my now girlfriend and I are 18. We all started hanging out and we got along great, had lots of fun. The girl had just broken up with her previous boyfriend(unofficial) of 6 months. One activity we got into was going out drinking at nights. Her and this other guy get along really good and though I've kind've been aquatinted with her longer, they have been close friends longer (maybe 5mths). Around the third time we went out to drink, my now girlfriend got very worked up while intoxicated about her ex. This other guy did a better job of comforting her in the moment and she ended up kissing him. Later that night they made out. I had figured she wouldn't remember it and we'd all move on from there. She claimed the next day she had no idea and was embarrassed. Being I had developed feelings for her over the time we had all spent and her and I's extensive texting back and forth (little had I realized it was just as extensive with him) i had brought it up again. She said "idk maybe I wouldn't mind kissing him". Low and behold next time we hung out at night it happened again. It was like I was t there. A few days passed and I finally told her about my feeling for her and how I couldn't continue hanging out until I allowed these feelings to pass. (Really I couldn't bare to watch them). Anyways we ended up going out again a couple days later. It started out she wasn't talking to me, she seemed angry. After a couple drinks she said it was because she had a hard time talking about feelings. She said she felt bad that I felt left out and she ended up making out with me that night. We got more drunk and it started to te point where she was going back and forth between us. He being bolder had his hand in her pants. I started to sober up and requested to go home (took one car). They brought me to my car and they remained in te parking lot (don't know for how long after). Days go on and he admits to her having feelings as well. She not knowing what to do did nothing for a while. We didn't drink for a while either. Basically we all hung out for a While and things went on ok on the outside, him and I didn't fight or openly compete. But deep down we were both wondering and worrying constantly. A tweet was posted one day by her saying head over heart. Now I being the safe guy who like long term relationships and previously being in a 3-year relationship, was the head (smart choice). He was the heart she to me at one point that he was a current (she thought crush) and that I was better for her. Problem is is that she seemed so happy with him. She was with me too but even more so with him. I was shocked at her decision (so was the other guy). So she chose me even after a week of me being out of town and them sorta going on a trial date. We did a sorta trial period for a week before him and her as well. It went very well. Anyways after telling him her decision he convinced her it was too early for her to be in a relationship. So she decided to wait. But told me that her decision hadn't changed. We continued as friends and in that subtle but still existent competitive state. I was unsure because they'd hold hands and be kinda couple-like just as well as her and I. Though her and I more often. It went on like this for a while. She slowly started to give more and more hints that she was favoring me for quite a while to the point where he'd get obviously angry. We drank again, heavily, and damnit it happened again we ended up getting very handsy with her. (Really she gets handsy when drunk, this other guy always took advantage and always physically bought the alcohol(though we paid)) . She viewed him as her best friend (so she says). Next time I told her that if she truly chose me then she has to stop this basically passing herself around to us. She said she would behave. She for the most part did, though still had her typical flirtatious attitude. eventually she finally decided she just had to tell him straight up again that it was me she chose. (Mind you this whole situation was crazy to me and I was extremely embarrassed that I allowed myself to be sized up and compared to another guy for her choosing). Now this guy basically got pissed and felt that she had made the wrong decision him being her "heart". And showed his true colors and even after us saying we wouldn't split up as friends, he stopped talking to her for a while. Turns out he told her he'd wait for her. Low and behold he's back again and they're talking again. They have an on and off relationship and this last two time they stopped talking was due to the fact that she found out he had lied to her about only sleeping with 3 women. (This girl btw is a virgin) he also started to see a girl who already had a boyfriend (another friend of his). So basically she realized that he really is a shitty guy. We have been dating now for 4 months without basically ever seeing him outside of work. We had even left that place of work entirely recently. Suddenly she replied to one of his messages again and this got me worried, not only because they may become friends again but because she apparently couldn't let this guy go. We have had a previous large fight over this and how I couldn't ask her to not be friends with him. I left it alone because it seemed to be ending with him anyways. Recently I admitted to snooping on her phone when I had seen a new message from Snapchat which I thought could be a new account or something. Now she has never given me a reason to believe she'd cheat but all of this previous info and more gives me (I believe) plenty of reason to worry. This became a huge fight between us and we are now recovering but it ended with it sounding like she is going to become friends with him again. And expects us all to hang out again. She knows I don't really like him, that I have past trust issues, and that he isn't a good guy deep down. My question is is do I have the right to ask her to not be friends with this guy again?? I mean she has gone to his house before when we were unofficial while I was at work because she was angry with me.. she didn't do anything and I truly believe her. My gut says I shoulda never started this whole thing and that I should dump her.. but we've gotten to te point where we tell each other we love each other, her and I spent every waking moment with each other this past month. I was wrong for snooping (didn't find anything) but I mean I really don't want to allow this whole thing to start up again.. what if they drink without me one night? What if she gives him the wrong idea? I think by inviting him to hang out with us she already is giving him ideas. Basically we're recovering from a fight over 1st) were spending too much time together and need space. 2nd) even after four months I'm still nervous about this guy coming back. Him going after her is the main reason I have problems with giving her space. We really do love each other and we have been doing great. She says he's just a friend but I'm afraid that now that we've gotting past that puppy love phase that he being back and still exciting may be able to take her away from me. Or that something may happen between them that'll hurt me deeply (also be the final straw). So as it stands I'm writing this asking do I give her the ultimatum..? Or do I just wait and see where it goes... I now have no way of knowing what They talk about as her phone is off limits to me after snooping. She told me she loved him but as a friend, back when we first started dating and he left she said that she chose me because she didn't want to lose him so she didn't want to date him... now that's all supposedly different knowing that he's a bad sorta guy...but she wants to be friends again... it's late and I'm typing like **** and probably left out details but please help...dumping her straight up isn't an option but an ultimatum is an option in my mind. Do I make her choose or wait it out??? Really sorry for the novel but please help!!!