Dude, seriously, you have evey right to feel jealous about that! She had a relationship with him while you two were together and cheated with him! What the f*ck? You should have accepted that things weren't going to work when you realized that you constantly fight. Also this "he's a friend" shit is just that...SHIT. She's still dating this dude right in front of your face! If she f*cked him once while you two were together she'll do it again, especially since they still see each other regularly. What, do you think they both lost feelings for each other just because YOU popped the question? NO NO NO NO. What you did/said has no bearing on the feelings that they have for each other. Do you have a ballsack? Does it have hair on it? Do you have a hotdog shaped piece of meat between your legs? If the answer to those questions is "yes" then you need to stand up, and tell her to get the f*ck out until she decides EXACTLY who she wants to be with. I really don't suggest that you do that though, as she's already proven herself untrustworthy. If you MUST, go with my first suggestion. If you've got the coconuts to stand up for what you know is right then give her the heave-ho since she's being unreasonable and inconsiderate.
...one can be sure of nothing until it has already happened...