Using a throwaway for obvious reasons.  I have an ex-coworker (from 7 years ago) that we've remained "friend" on that social media platform that is used to keep in touch. I mainly use it to keep in touch with all my old Army buddies since we're scattered throughout the world now.

Anyway this previous co-worker has always had some issues with mental health.  Not just depression or anxiety but some pretty severe PTSD as he's a former Military person himself and he's also been put on an extended hold for some pretty serious thoughts (violent thoughts towards himself and others). He's been abusive to both his ex-wife and current wife.  He's also not allowed anywhere near his children from his ex-wife due to his abusiveness.  Which in the state I'm in the judges always award 50/50 custody unless the other parent has been proven to be extremely abusive. Even then parents are given a chance to be rehabilitated to be able to keep joint custody. Parents that have serious drug addictions even get to keep custody of their kids. I'm only giving this background info to show how badly someone has to fuck up in my state to lose custody of their kids, it's rare.

He has been married to his current wife for about 6 years. As soon as he divorced the ex he immediately (in about a month) jumped into a relationship with the new wife.  At our previous job he never asked me out.  He did while he was going through his divorce and we no longer worked together. I politely declined and he seemed fine with it. 

Fast forward to three months ago. He sent me a message on the dating website I'm on. I ignored it.  Well last night he sent me another message and I asked him wtf he was even doing on a dating website since he's married. I got no response.

So would I be the asshole if I told his current wife he is on a dating website?  I know they're still married and just two days ago posted on the social media site how she was the love of his life and blah blah blah.  On the one hand I don't think it's my business and it would also piss him off.  But on the other hand I'm not the one that's married and trying to date other women (his dating profile states he's divorced and looking for a LTR as well as quite a few made up things about himself).

I would just like some opinions on if I should say anything to his wife or not. If it was me, I would want to know. And maybe that would be the eye opener she needs to leave his abusive ass. Idk, I'm torn. So please help me out here Reddit!