Yucky...
There are 2 things that I want to mention here; firstly, I have a paragraph dealing with illness that says that if a child becomes ill in my care they must be picked up within 1 hour of my communicating with the parents. Now, of course, I would have some flexibility. But the point is: the child is NOT allowed to stay. In future, I would maybe be more tough on that point.
HOWEVER, my second point is: that's how it is in multi-child care sometimes!! If you want to ensure that your child will never come in contact with these germs, keep them at home. All the time. In a bubble. LoL
I would not worry about that mom. Explain to her that you did what you could, as soon as you could. Chances are, the kid was communicable well before you knew he was ill.