I don't think there is anything wrong with telling her how you feel, but if she decides to date you you had better watch out for the other guy. She obviously has some unresolved feelings for him, and may very well go back to him while you two are in a relationship. If it were me I'd ask why she was going to go back to him, and let her do what she wants. She may NEED to get her heart broken again to get it through her obviously thick head that he is a bastard.
...one can be sure of nothing until it has already happened...