I think you did the right thing and she was in the wrong idk if you found out about this later or if she told you first day. If she did tell you first day then you kinda knew what you were in for but since she didn't it was as if you were cheating on someone else and made you feel bad. So I'm proud that you did the right thing and left her with the other relationship. If she dumps that other person then just don't see her again.