Growing apart from friend over her cheating

I have a girl friend that I've known about 3 years now. We hit it off right away initially, but for about 9 months now our friendship has transformed, and not in a good way. This is gonna be one of those long stories I'm afraid..

The thing is, she got married 1 1/2 years ago. It was a whim decision, even though they had been dating a few years. A few months after the wedding she got into a bit of an identity crisis over realizing that she has bound herself to this guy. She started talking about a certain ex, who had left her about 10 years ago (he doesn't even live in the same country, and they are different nationalities). Right before she got married he had texted her something like ' I still love you and want you to break up with your boyfriend'. When she sort of gave him the chance to come and steal her away, he never replied again.

Anyway, a few months into the marriage she got into her head that she needs to see him one last time to get some closure. She did a pretty good job at convincing me that he's just a ghost from her past and she feels like a good chat with him would solve her current marital issues and clear her head. She asked me to go on a mini-break to London with her, and she could just go for a few drinks with him at some point and talk with him. It seemed safe enough, especially as he was apparently in a committed relationship as well.

They met up for drinks, talked about a few hours and then I joined them, as per planned (sort of as a cheparone, I suppose). Apparently they hadn't talked about their relationship at all, but were instead just catching up, and the three of us had a pleasant evening. We decided to go clubbing, and had fun. They were flirting a bit, but the guy was also flirting with me and I sort of assumed that was just his personality. At some point, he drunkenly invited us to have a 'threesome', which we laughed off.

At one point, when I went to the bathroom in the club, I came back to see them making out. I got so mad, and got into a fight with my friend, and we went home. I blamed her for being irresponsible and selfish, and told her very strictly that I didn't approve of her behavior.
When we got to the hotel, I went to wash off the makeup. When I came out, she had disappeared, and had turned off her phone. Of course, it turned out that she had went to her ex's hotel and spent the rest of the night there.

Now, to this day she swears that they didn't have sex, but I am absolutely certain that they did. When we were fighting, she had claimed that 'in her heart she knows she should be with this guy instead of her husband, and they should get back together'. She also blamed me for being untrustworthy as a friend, which hurt a lot.

We eventually did make up, and I told her that what she did was not really any of my business, but I just felt bad for her husband (who's a really nice guy and totally in love with her).

After coming back home, she kept texting her ex and they exchanged 'I love you's' etc., which she told me all about. I told her I felt uncomfortable with the situation, but out of fear of her shutting me completely out (or blaming me for being a bad friend again), I listened to her situation. Apparently for a time she seriously considered that she would leave her husband for this guy, but I don't think he ever truly felt the same way about her. He was, after all in a relationship, and had probably just wanted an extra-marital swing with my friend, not a new relationship.

The problem is, that ever since this incident I have seen my friend in a totally different light. She used to represent to me a person who knows what she wants, and is very sensible. That image totally trashed on our trip, and has been replaced by general untrust. I feel used, as it seems to me that she definitely knew what was going to happen between her and her ex, and used me as a tool in her plan (obviously she needed a good reason to go to London and a girls' weekend was a perfect excuse). I also feel that she included me in her extra-marital affair to sort of 'force' me to become her ally if she would break up with her husband.

I feel that if she is so able to coldly manipulate and lie to her husband, she can do that to anyone, even me. I am also irritated over the fact that when I had a hard time in my current relationship, she pretended like she knew exactly what was good for me (offering me these ridiculous 'rule' books for women about the rules of dating), and claimed that she never breaks these rules and has lived a successful dating life. Now I feel like a fool for ever believing a word of relationship advise that came out of her mouth.

I also hate the fact that she expects me to hang out with her and her husband, even though I feel personally so ashamed for her behavior that I can't really even look him in the eye. He's a really good guy who's been supporting her both financially and emotionally, and I feel that she is using the worst kinds of double standards with him. he's not even allowed to have any female friends, and not a lot of male friends either, and my friend insists she's 'training' him to be the perfect husband (including paying for everything etc.)

I used to think she was just insecure because she has once been cheated on, but now I think that she's just selfish and egoistical.

Anyway, we have started to grow apart because of this general lack of trust on my part, and my friend has became bewildered over the fact that I am not completely 'hers' anymore (I have started seeing a lot of my other friends as well).
The thing is, I had just moved back to my country when we met, so for a long time she was virtually my only close friend, and due to my problems in my own relationship around the same time, she became very important to me. Because of this, she had maybe a bit too much 'power' in our friendship (as I was quite dependent on her). I think that she is the type of person who likes to use this kind of social power over the people in her life. Unfortunately I have been unable to analyze her this way in the past, instead always just admiring how she had 'a way' with people.

I don't want to lose her entirely, and she definitely doesn't want to lose me. In fact, she has been questioning why I have been so distant from her lately, and is probably under the assumption that my distant behavior is due to the fact that my grandmother died recently, and because I moved in with my boyfriend.

But I am having a hard time talking with her about this, because she gets very defensive if I bring any of this up. I also feel that if I do bring her behavior up, it will possibly completely ruin our friendship. At this point, I also think that it's not just her 'affair' that's causing the friction in our relationship, but instead the fact that it has only recently dawned to me what she's really like as a person, and I am not coping very well with the realization, although I realize that it's partly my fault for having seen her through 'rose-colored glasses'.

Do you have any suggestions, ideas, anything to say about this situation?