My MIL is having an affair with a married man...

Long story short my MIL has been talking to this man for 6-7 years. Well, recently my husband and I posted our bed on Facebook marketplace and started receiving inquiries about that.. well, my husband had message come through on Facebook and asked if I could respond. So as I’m searching for the message since it wasn’t in his inbox, I checked “message request”. I found the message there and responded to the person that was interested in purchasing our bed frame. Since my husband is not really on Facebook I decided to see if we missed anyone messages from anyone regarding the bed frame and that’s when I found a message from 2015 from the wife of the married man that his mom is talking too that said “Hello, you don’t know me but I just wanted to say hi to one of the sons of the woman that has been having an affair with my husband. Yes my husband. Tell her that there’s 2 sides of the story and now your mother is the side chick”..

Later that night my husband calls his mom and ask if she knew the (wife) except called her by her name and his mom says, “yes, I know her that’s his ex wife” his mom then said “yes” she was talking to the married man WHILE he was still married and then asked my husband to forward her the message he received and goes on to say that she will talk to my husband in private about it... I just can’t believe this! No, I don’t have nothing to do with it, but when you try to carry yourself as a Christian, etc this should not be happening, Christian or not.

I’ve also sensed jealousy from my husbands mom on multiple occasions ( too much to type LOL) I have no problems with my MIL but i kind of look at her differently..... am I wrong?