I'm assuming there is more to the story than your young friend shared. She said "it's about a pattern of behaviour that shows a lack of respect for our boundaries". If the MIL is constantly trying to get the friend to reduce her time with her own family so that she can get more of her grandchild's time, and is constantly being passive aggressive like texting every single day "to make sure they're ok" (which is a fake reason, the real reason is to pressure them to respond within 1 day, saying it's "to make sure they're ok" is a cover story to make MIL look good) then I could see how this example would be the final straw.

I highly doubt it was purely based on what happened at Easter.

Also you say "it's an easy equation of the good outweighing the bad" regarding your own MIL. However your friend may have a different situation with her MIL, in her case the good may not outweigh the bad. She's not cutting off contact but she's reducing it to try and keep MIL from overwhelming her.

I agree that you shouldn't use boundaries as a weapon and you should give grace though.