>it is not your fault

Part of it IS his fault! He did not stand up for himself. He did not demand that his girlfriend behave honorably to him. He acquiesced in taking the friend along to J's home, even though the friend had already betrayed his trust.

Hopefully, he will learn the lesson from this experience, that a man must be strong and must protect his feelings against toxic friends and lovers. When he realized his girlfriend was unworthy, painful as it would have been, he should have rid himself from her forever! Same with his so-called friend. None of us should allow ourselves to be so weak that we would tolerate such disrespect.