What do I do about my girlfriend's ex boyfriend?

Well first of all, if she's lying to you about the fact that she still is talking to him then you should get rid of her. Obviously she's a liar and if she lies about talking to him then what else is she hiding from you? I dont think its ok to control who your GF hangs out with or talks to but its odd that she still wants to see him or talk to him after they have broken up, especially if he cheated on her. Just be careful with her.........She's already lying to you!