If MIL-to-be starts calling at 8am, and doesn't stop til 8pm, that's a total of about 2.5 to 4 calls PER HOUR.
If DIL is not exaggerating, I would strongly suggest she put a call-forward on her phone for just one day, and forward all calls to her fiance's phone.
Bet Mommy would be told to cease and desist pronto!

Oh, and run like the citizens of Tokyo fleeing Godzilla, whether or not fiance wakes up. He won't stay woke long.