yeah, the best advice I can give is not do anything, and leave it completely up to her. That way you're blameless (to a degree) if any shit hits the fan. The guy will be suspect if he gets dumped and she hooks with you, so give it a few weeks, and if the dude hits you up, just say there was no pretense while he was with her. Thats all bases covered I think.
Also just by getting involved in these thoughts you're probably carrying too much baggage about the situation already. What Tsunami said, its her situation, you DONT want to get involved with a chick who is gonna make her problems become yours too.
It happened to me last year, I hooked up with a chick in class who was cheating on her long distance boyfriend by doing so. She said she was fine with it (she initiated everything), but then she'd turn around and treat me like absolute shit, like I was the bad guy and coerced her and shit. I didn't have a clue what was going on, basically I just figured she was unhappy with herself and trying to pin all her shit and issues on me.
At the end of the day, life's hard enough without carrying someone else crap, you gotta look out for numero uno