<human>: Could you distill the essence of the text into 1-3 sentences?

My mother has been seeing his father for a while now and weve been friends since about 5th grade(high school seniors now). So in the past he has had sex with the girl that everyone knew that i was seriously talking to and now i have a hunch that he is going to try it again. I no longer trust him, but i am normally a very passive, forgiving person. We never resolved what he did in the past and if he pulls this shit again its gonna be ugly and end up as a "family" conflict. Im about to go off to college but he will be there too. How should I go about confronting/ resolving the situation without breaking our friendship?
<bot>: Friend hooked up with girl i was talking to in the past and  i am worried that it's going to happen again. Must resolve without breaking the friendship.