V.W.
This doesn't really solve the problem, but maybe she shouldn't be going with your son to his dads? If she needs a place to stay, and you are providing that, why does she go to his dad's place? I understand you might enjoy the time without her, but if you are only allowing her in your house b/c she needs a place to stay, why would you allow her to travel with your son? Would you find it acceptable for him to stay at a friend's for a weekend sleep-over where girlfriends were also allowed? Treat her like you would any other foster child you took in.. (YOU took in, being the operative words.. your ex did not take her in).... You might get to know her better if you are with her when your son isn't around.. and she might decide that it's not as much fun and make other living arrangements on her own. I also agree with the poster who suggested asking her parents why she was "kicked out"... Are they horrible parents? Are they monsters? or do they know their daughter better than you? If they ARE monsters and abusers, then you are a lifesaver to this girl.. but you are not doing her favors (or your son) by allowing them to visit his dad overnight together. She should behave (and be treated) like a foster child or a tenant of yours (not your ex's)..
Best of luck...