My FIL is/had an affair 🤔

So, I will try to make this not so long, but I will need some advice because I’m torn on what to do.

Tonight I had a message pop up on my Facebook messenger from my MIL’s employee asking about a Christmas gift for her. I noticed I had another message from someone named Judy that I don’t know & it was from October of 2015. Like why did this just pop up?! Anyway i read it & basically this woman is saying she’s been with my FIL for a while & wants to meet me, my husband, my BIL & my daughter. I showed the message to my husband & he said she must of had the wrong person. There’s just too much detail in that message for me to believe she got the wrong person! So I went through all of her pictures & sure enough there’s some cuddly posing pictures of her & my FIL. Now, this was 2 years ago. So do I just act like I know nothing? Do I say something to him? Do I tell my MIL? Idk what to do. My in laws & I are closer than my family. I’m honestly hurt by this like I feel like it’s my father that was cheating on my mom. Opinions please!!!!!!!!