I would put my foot down big time.

I'd her in no uncertain terms if she doesn't give crap about his health, and puts him in danger, she isn't welcome near him, and what a disgraceful grandparent she is.
let her know that him catching an infection could of been extremely serious, and she put him in a lot of danger. I would even end it with "how could you be so uncaring towards a little boy, your own grandson???"

But I am known to be very protective of my kids, and wouldn't stand for this.... so my advise might be a little over the top for some.....