Thanks for the responses, guys. It's helpful just to know that other people have had to deal with the same kind of situation. Yesterday she admitted that she had slept with him, but she assures me she's not going to get back together with him, she just wants to be "friends with benefits." Of course I don't believe a word of it, because this is exactly what happened the last time they broke up.
I really do think it's a self-esteem problem; she knows she can do better, but she's afraid she can't, so she'll settle. A few years ago when she and this guy were officially together, she was also dating a couple other guys, both of whom were much better. She gave excuses as to why she chose this guy over them, but honestly I think it's because she was afraid she wasn't good enough for them, so she opted for the piece of trash instead. I bit my tongue when they were together, but this last breakup felt like it was actually going to stick, so I was much more open about how I feel about him. So at least she knows what I think of him, and I'm not going to hide it anymore. She even admits that she hates him, yet she still continues to see him. That is all kinds of messed up right there. If she sees him again, I'm thinking of staging an intervention. :P
And as if stupid friend stress wasn't enough, I just found out my grandma is having a really difficult time recovering from her heart surgery a couple weeks ago, and was moved back into intensive care. And I found out the other day that one of the dogs may have cancer. NOT a good week!