People only get upset about boundaries being placed if they benefitted from not having them.
Your mother's concern for her grandchild's health is great, but once she was aware of how you are handling it she should have backed off.
If her only concern was for your kiddo's health, then cutting off communication doesn't make sense. If I had any sort of concern about my grandchild's health I'd be sure to do whatever I could to stay in the picture in order be able to help if I could.
I understand your mom is grieving, but...she's your mom. Mom's don't get to act like assholes even when they're going through a tough time. Moms don't get to be the cause of drama in their kids' lives, period.