This guy the girl snapping with could just be a friend and Are you guys going out now? Cause if you're not then you don't really have any say in this since she is free to do anything she wants since shes, not in a relationship with you but if you are, ask her about the guy also if there is no trust in the relationship re-evaluate it because there is no point being in a relationship if there's no trust. Also the drinking thing, it was probably milk.