Why does my girlfriend want to be friends with a guy who raped her?

About 4 years ago one of my gfs exs raped her.They were together on and off for 3 years. She told me the story and everyhting. He also cheated on her and treated her like ****. Now we've been together 8 months and she keeps wanting to talk to him/possibly hang out with him then a few days later she doesnt, this has happened 3 times now and she always says that i would be there too, she wants to talk to him because she says he knows the most about her and that she can be herself around him completely and she misses having a friend like that which is what he was before he raped her. She says she doesnt want to be with him nor have feelings for him but i dont know. Somebody please help me? I'm 22 and she is 20. This **** makes me really uncomfortable because of what he did to her and ive told her this but i dont know..