Honestly? I think that if there is a chance that he would leave this girl for you, there is that chance that it could happen to you as well. The fact that he is showing interest in you while he has a g/f may be a sign that this could be a bad situation. BUT this is not always the case. My advice to you is that if this guys pursues a relationship with you that you act with caution and take things slowly, make sure you really know this person.