Most Helpful Guy
It would have been one thing if his friend was having the threesome with his girlfriend's approval but I'm certain the girlfriend would have wanted to be involved too if they both approved of the idea. There's no reason to hold back the truth from her because while it may be painful and emotionally traumatizing, she'll appreciate the fact that you told her so she can decide whether to forgive him or move on. She's going to find out one way or another because men will behave differently when they are less intimate with their girlfriends when they start flirting and having sex with other people. If I was in her shoes, I'd want you to tell me as soon as it happened so I can be mature and take direct action on it.
Most Helpful Girl
I will live by this motto till the day I die do unto others as you would want done to you I'll never forget when I started my first year in college and my own mother and brother saw my then-boyfriend talking to another girl and taking her home and they never told me and I just felt really betrayed by them.. now you also have to consider the second case scenario there has been times that I have told on the person that was being cheated on and then I got made out to be the liar they ended up hating me both but I just had to tell myself hey I done the right thing so I was able to hold my head high at the end of it