While her having him around, fawning after her is completely unacceptable, so is you telling her that she must do X about it. You are not her father and you have no right to make decisions on what she should and shouldn't do in her life. It would be a very controlling move on your part. Not good at all.
Instead of dictating her actions, I suggest you simply decide whether or not you want to be a parter to someone who hangs out with a guy who's infatuated with her. Feel free to share your misgivings with her. In short, it's an unspoken ultimatum. If she can't understand you rethinking the relationship, then she's not the right girl for you.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.