<human>: So my bf cheated on me and when I found out, I left. But just before I confronted him, I had looked through his WhatsApp chats. I'm not proud of it, but when I came back from visiting my parents for the weekend and he had a hickey coverer by a plaster, I was obviously suspicious (note: i didn't know for sure it was a hickey until a day later, but the plaster was suspicious enough). 
What I found were messages with a girl who isn't even the girl he cheated on me with. It was a girl he had apparently known since he was 13, and who had invited him to join her while watching movies naked in bed. He also mentioned to her that he couldn't for "reasons", but never actually told her the reason.

I didn't bring her up since I was still very confused and shocked about the fact he has fucked someone else multiple times. 

I know this will sound super creepy, but I think I've found her on Facebook. So my question to you guys is should I message her to let her know what was really going on? Its not her fault he's a lying sack of shit and I'm worried he might take advantage of her not knowing now that I've left. He took advantage of the girl he did sleep with and I told her it had been behind my back all along (we'd had a threesome before and she thought if he was coming over that I must have said it was okay, there's more to it than that but it's too long to explain).
I know I'm not responsible for these other girls, but I feel like they deserve to know. Also he doesn't deserve to get away with it.

Any advice or insight will be appreciated. Sorry for any typos or bad grammar, posting on my phone is awkward.

Can you summarize the given text in a few sentences?
<bot>: left my cheating boyfriend, found a girl he was secretly flirting with who didn't know about me. Wondering if I should contact her to let her know the truth.