Have fun with this op... message her with "MUM??"
When you see her, huh her, laugh and laugh and laugh (making sure you don't leave your keys anywhere!) and then seriously tell her that this isn't for you at all. Be honest.
She needs to know her private life isn't as private as she thinks (she may not care), you need to find it funny that curiousity killed the cat!
Deal with this in the right way you and your MIL could actually become closer (as friends!) through this... so long as you're both honest. (and laugh with each other about it, take her for a coffee).
I am not a swinger, I am happily married and love HockingsVanilla. I also realise that some of parent age friends are still human and enjoy the ocassional sprinkles and flakes, I find it life affirming that although I crochet my mum's best friend, a grandmother, is a um, what's the word for a swinger when only one does it and the other goes fishing? (unbeknown to my mother).