For all you know, she's very unhappy with her bf and is looking around for something else. It's not your responsibility to protect their relationship. I'm not saying you should TRY to break them up, but avoiding her completely just because you think she "belongs" to someone else is silly.

I think you should make yourself available for her to approach- sounds like she's not shy, so if she wants to, she will. You don't necessarily have to say, "Hey, let's talk for a few hours." It will just happen.