Should I tell his girlfriend?

There’s this guy I’ve been hooking up with 10+ years ago. He’s friends with my husband of 9 years (we’re going to through a divorce right now so I’m basically single, even though I don’t think he knows that ). I posted a picture on Facebook of my daughter and three kinds Day. Out of the blue he messages me, we haven’t talked in those 10 years. He apparently had cancer for a while hbso I asked him how he’s doing etc. What he’s referring to is from 10 YEARS ago!

See conversation below

I’m not even going to respond again, but I’m wondering, should I tell his girlfriend? I know who she is, I’ve even met her through mutual friends, and they’ve been together for about 6 years while she has been supporting him though his cancer battle. I don’t want to be the asshole, but really???