Months have passed. Initially, I had thoughts of telling her, but I didn't want to get involved. So I decided not to. A few days ago, however, a close friend of mine became the victim in a similar situation. This made me think of my own experience. Maybe I should tell the girlfriend after all. I feel guilty. What would you do if you were in my shoes?
I would stick with your original plan and stay the hell out of it. This girlfriend of his is not your close friend. You don’t know her, and you’ve never spoken with her, so it’s not up to you to figure out who’s telling the truth and who isn’t. Maybe everyone is telling the truth, and it just doesn’t look that way on the surface. Photos rarely tell the whole story, and social media is not reality. Even if you saw photos of them ravishing each other in front of the Eiffel Tower, it doesn’t mean they were exclusive. What happens in their relationship is between the two of them.
I recommend spending your emotional energy on loving and healing your close friend, not on soothing your misplaced guilt. You didn’t do anything wrong, and there’s a very good chance that nobody was misbehaving. Shift your focus away from the happy couple, consider the situation a part of your past, and then carry on with and tend to who’s in front of you now.