Most confusing situation ever

Okay so my last boyfriends ex is coming to me and venting about him and how much he hurt her. My ex hates her and thinks she's obsessed and stalking him, to put it plain and simple he dumped her because he had no feelings for her and didn't want it going on any longer because he felt it would hurt her more in the long run. So this girl and I have been talking for 2 hours and I'm trying to tell her move on because this is going to destroy her. I can't be mean to her because I'm just not mad? I don't have a problem with her and since I've been so nice she's like confiding in me. He's been so mean to her and texts me horrible crap about her so I told her, you know you have this hope that he'll love you one day so you just wait around and let him treat you like shit until then? I don't get why of all the people on this planet she chose his ex of 3 years to confide in. What does this even mean.