What should I do about him?

My cousin(mike) is 18 just out of high school. He lives with us for certain reasons. His girlfriend(lets call her sally) is really nice, polite, and never is rude to our family. But tonight at like 1am he brought home another girl. He just took her home. When I went upstairs to his room, I found a freshly USED condom with fluids on his nightstand. I took a picture of it for evidence. He's cheating on his girlfriend of almost a year now. Should I let him know that I know? Should I keep it a secret? Should I talk to his girl friend about it? Help please! Oh and they're not in a casual relationship.