I agree ... especially if you care for other kids, you have a repsonsibility to all of them. These symptoms are not "normal" and they aren't going away. Let her know that you have to follow state and/or CDC guidelines, and that he won't be able to come back without a Drs note, or until the goo is gone and no fever for 3 days.
It may be that she doesn't have insurance. You could go the extra mile and look up if there are any places in your area where she could take him for low cost - it might help (if you don't think she'll get upset at you "butting in")