You mean one of those "sitcoms" that is actually a sad bummer like "Kenan?" Man, I expected so much more from that show! Making Kenan the straight man was such a waste of his comedic talent.

When life hands you a shit sandwich there’s usually not really a lot of choices. If it’s any consolation at all - I doubt MIL thinks much of what her daughter is doing and she’s likely to bring it up to her at least occasionally for the rest of her life. But this is not much consolation… (1) See a lawyer and understand what your options are. The first consultation is usually free… (2) When the psychological shock wears off, you will need someone to talk to. When my Dad was alive he’s the one I talked to. Sometimes work offers a free counseling program for employees - I have used that before… By the way, while it may be tempting talking to MIL because she’s there, you know her, and she’s Grandma. Eventually MIL is going to feel forced to support her daughter or just “caught in the middle”. You are going to need someone to talk to who is either professionally neutral or who is on your side. Make the appointment to talk to a lawyer ASAP.

time to lawyer up, and go nuclear. Her fling with her boss will end poorly for her, and she will try to come back to you. But she belongs to the streets! It sucks, and you'll have your heart ripped out many times for the next little bit, but just focus on rebuilding you and your life. One day she will come crawling back, when her life is in shambles, and her new man trades her for a newer model. And then you can put her in her place, with a nice smile on your face. Good luck!