I concur with Mal on this one.
Quick opinion : she might feel bad for her ex, and so might you - but...if the two of you are in a relationship and have made that known, and the ex isn't willing to let it go, then I think your girlfriend has to make a choice. It's not easy, but sometimes that's what it comes down to. If she's isn't willing to make that choice on her own, and hasn't been willing to, then you should pay attention to that.
You can't make her make that choice, and this situation sounds like a recipe for tough times. You can't force things to be how you want them to be, so in my opinion I think all you can do is be aware that something is amiss and be prepared for things to get dicey, or else cut and run. If she's aware that her ex's attention is causing you problems, and she isn't taking steps to make it clear that she's not interested in receiving that attention, then you've got a problem.
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