well, as the thread title asks, I can tell you what I'd do, and why, but as for what yOU'LL do, well, you're gonna have to decide that on your own.
Me, I wouldn't touch something like that with a 10 foot pole. She's made a commitment. whether she likes it or not. if she decides it's not for her, she needs to break up w/ the fiance, free and clear, before she starts something new. I've got some stiff personal morals against cheating, and helping someone else cheat. It's just cruel, and i won't be a part of it. Plus, that keeps me free and clear from some jealous girlfriend who wants revenge for someone stealing her man.
So, I'd have her over, show her cali, and leave it at that. if she needs a shoulder, thats great. if she needs a cock, I think she should find it elsewhere.