The fact she even mentioned it to you does have a bright side. While it does show she still is thinking about this and the potential situation it may cause, it also shows that she's at least honest about her feelings. If she was deceptive and planned on just dumping you at a moment's notice for him, it wouldn't have even come up.
I would say (or rather my psychologist wife would say) the next time you see her, talk to her about it. Say to her that you've been thinking about what she said about her ex and what it means to her. Bring up the point that the fact she even mentioned it makes it seem like she thinks there might be potential for something to happen (she never would have said anything otherwise). And let her know honestly how you feel about her, but you also need to know where she stands.
Good luck - kinda of a sucky situation though. =/
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well this is definitely not what I wanted to wake up and see - Reg