Please help

I've been divorced for 2 years, own a business the past 3 years with my best guy friend of 18 years. He's married. Our families were friendly for many years thru our kids.
As friends, i know his marriage has been on the rocks for at least 8 years, past few years in particular have been horrid. She's very difficult which is why i ended our relationship years ago. I can't be friends with her, she is not the nicest person.
9 months ago, he admitted his feelings for me. I have always had feelings for him and admitted mine. started an affair. There's Always been an emotional affair going on with us. She questioned him a couple of times in the past about our relationship, nothing was going on at all and he said that, she's dropped it.

After 2 months of just making out and all that stuff, it became fully physical. We own a business together and see each other a few times a week. We talk at least 10 times a day and text througout.
She has no idea- i dont think. If she does, she doesn't care.
She hasn't had sex with him in 4.5 years, again, i know this bc he'd ask for advice when i was still married and we were simply best friends. He said she says she doesn't care if she never has sex again, not interested. They also dont communicate. It's a roommate situation and a bad one at that. He feels emasculated - he should.
He sticks around for his youngest - 16 yo and has said for YEARS (when we were just friends) that as soon as his youngest is graduated and in the military, he is out of there. They live a "fairytale life" from the outside - all the stuff, the friends, etc. But those close to them know the truth, it's a nightmare.
When this all happened, we discussed what to do from here. Discussed doing this until he graduates and then he will leave her. Not entirely sure about that - i think he's scared of what others will think. He's fed up with her and thre are days that her fights with him are so horrid (I've witnessed them) that i can't believe he's sticking it out. HE's afraid for money, for stuff, for the unknown, etc.
We dont have that conversation often bc i dont really want to know what i dont want to know. I'm afraid that he may not ever leave. By the way, he's a total guys guy so just that he seems to just put up with her makes me shake my head.
We truly do have the best relationship and i want this to work out.
We talk about Everything, of course the sex is amazing, the friendship is solid, the connection has always been there.

Our families (kids) are all the same ages, all get along great. His kids keep asking him WHY he stays with her (i've heard them ask it many times!), she's just a Bitch to them all.
My question - WHY would a guy who is treated this way at home (and by the way..again, i know these aren't a bunch of lines..i know and see first hand how it's all real.) and not just leave especially if he and i have discussed after it's over and being together?

I find it VERY hard to imagine that he's feeding me lines...with a business and long friendship at stake. I know his feelings for me are solid, as are mine. I just dont know how i'm going to do this for the next year and a half. He says he feels bad when he does things with his family - which by the way are rare with her bc she can't be bothered...but they do go for dinners on occasion and i do get jealous but i dont tell him that.

Also, a couple of months ago - after 4.5 years of no sex..she came onto him. He turned her down and she threw a massive fit..to the point he called his best friend after (who i am also friends with ) and filled him in. (His Best friend doesn't know aobut us). He told him ... 4.5 years and she decided to try, i couldn't get it up even if i wanted to. She doesn't turn me on, i dont love her and i didn't want to. She had a massive fit that he doesn't find her attractive or any of that. He blamed it on 'guy problems'.
I honestly think it was a test. I think she maybe a little bit was interested but i think she thinks perhaps something is going on and was testing him.
I dont know.
Just help on all of it.
I do NOT want to end this. I do NOT want to give it up. I'm just trying to understand a guys point of view and get some advice on how to swim in these waters.