SUBREDDIT: r/relationships
TITLE: Cheating - would you want to know?
POST: If your SO cheated behind your back with other women/men, would you want to know? Or be left in the dark? 

If you knew of someone that fucked with other girls behind his girlfriend's/wife's back, would you tell the SO that didn't know? 

Background to my question that I hope to get some kind of insight into: 

My SO and I split up [both mid 20's]. 
We are together again now. While apart, she left her FB logged in on my computer, and like an asshole, I read her messages. Discovered some guy messaging her sexual flirty things and that his girl was out of town etc...she went with it (damn, still hurts to think about). 
I don't think my SO knows that I know...
I contemplate emailing the guys SO very subtlety...I don't know though, I know what it's like to be on that end, and I don't want to hurt someone. But I also don't want to sit back knowing this coward can get away with this shit. I'm torn. 

What would YOU do?
TL;DR: 
if your bf / gf was fucking with people behind your back, would you want to be told? Would you tell if you knew?