For context this person used to be one of my best friends. We were roommates for awhile and he was a great buddy. 

He had been dating this girl for over 4 years when this first started happening. I was away for school and hasn't seen him I awhile, and when I returned for a visit he told me.

He had cheated on his girlfriend with another woman multiple times. This woman happened to be married to someone his girlfriend knew. He ended up getting this married woman pregnant, and he and her split the cost for an abortion. He then basically cut contact with the woman and his girlfriend has absolutely no clue that any of this has happened.

This guy is friends with most of my friends. They all like him better than me. Him, his girlfriend, my other friends etc. are all part of one big friendship circle. When he told me this story I got angry with him, told him we weren't coop anymore, and basically don't speak to him anymore. 

I told one friend about this story and he agrees it's fucked up but he told me not to tell anyone. Neither of us have seen this guy around anymore but we've seen his girlfriend once and it's clear they are still together and that she has no idea.

I feel like telling the truth would be like dropping a nuke in this entire circle of friends, especially since it has been almost a year since he told me this story. 