she is probably in love with the guy, if they have lived together and been friends for 8 years they obviously have a connection if attraction came into play, it is pretty likely they are confused as to why it happened due to the platonic nature of their relationship - but the fact it did means there are probably feelings involved. you are the buffer, in my opinion and need to leave her.
I'd say that you offer for her to have a romantic relationship with both of your, or keep him as a friend with benefits. She may not like him enough for him be her primary partner, but she does have some sort of relationship with him. It's up to you if you want to share her. I'm in an open relationship right now. It's been difficult at times, but it's been worth it.
Pretty sure she doesn't. You said she fell apart when you talked about seperating. She's emotionally manipulating you, and pretty unstable herself.
Being really upset when someone tries to break up with you isn't always an idicator of being unstable. It might just mean she really loves him. None of us have enough information to judge here. Yes, she shouldn't have cheated on her fiancé, but people make mistakes, even big ones, and it doesn't mean they are unchangeably bad. We don't know this girl. All we have are a few sentences from her boyfriend to go by. She could be a nasty, selfish, emotionally unstable cheat or she could be a decent person who made a mistake but loves her boyfriend. I also couldn't speculate on the room mate thing. If you don't trust her enough to believe her when she says there's nothing between them, then yes, you have a problem. You need to talk about all this with her.
If you're sure she loves YOU, GuitarMann, then go for it. And even then, keep an eye on them, both of them... One can never be too sure...
GuitarMann do you know what kind of a person her ex finace was? Was he a shitty boyfriend? It's a possibility that perhaps she cheated on him because he was a terrible partner. Sometimes people are "driven" (for complete lack of a more suitable term) to cheat because their partner treats them like shit.
Recipe for disaster. She's a cheater. She has feelings for the other guy. She lies to you about important things. Do you really NEED to wait until you catch her in the act of cheating on you? I think you can see where this is going. There is a pattern here.
Dude I strongly advise you to get the one up on her and dump her NOW. I was in a situation very much like this, involved with a girl who lived with another guy and they had an oddly close relationship where they slept together but "didn't" have sex. Looking back on that I feel fucking retarded. From what you're saying I'd maaaaybe agree to live together WITHOUT the other guy but like everyone is saying it's still likely that there are some feelings between her and this dude and it's eventually going to drive you nuts if you let yourself fall for her.