Fake names, so it's easier to understand:
My son: Bob
My son's girlfriend: Alice
My best friend's son: Chester
They're all 18 and 19. Bob and Chester aren't friends.
My best friend told me about the first time Alice slept with Chester, he confessed to her. She said that Alice and Chester have been liking each other and have been seeing each other. They've been having sex for 8 months now. I'm not sure as how to deal with this as I don't want Bob and Chester to end up fighting, because Chester is stronger than Bob.
I've been looking the other way this whole time but should I get involved?