A schoolmate invited me to drink together. I knew he was sort of going out with a girl on that time but I also knew that they weren't in a relationship, so I went and hooked up with him. Few days before that he invited himself on my birthday party and told him that he can come. We hooked up with each other the whole night then. But the next morning the girl he was going out with wrote me. It was about something else but I realized she loved him and I told him that we mustn't meet each other again because I really liked the girl he was going out with. Now he doesn't speak to her because he blames her for not going out with me and she is being very upset now and hates me, but thinks that it is her fault because she was jealous and he had told her that between them is nothing serious and she has no right to be jealous. I know that he is behaving like a jerk and that I made a mistake but I really don't know what to do.