OK so I've been going out with this girl for some time now. Everything was perfect like 3 days ago, until this guy she had liked started talking to her and telling her he liked her. But I know for a fact the guy isn't being sincere. I really love her and don't wanna lose her but I don't wanna be in a relationship where she doesn't even trust herself talking to the guy. For now I've given her some time to think but I figure she should end any contact with the guy and if not just end the relationship and prevent myself from getting hurt any further any suggestions on what to do or tell her?