Watch for dehydration and for a temperature. Anything over 102 should be treated with antibiotics. We prefer Gentocin for foals.
Pinch the skin on her neck and if it stays puckered up, she is dehydrated and needs fluids / electrolytes.
A probiotic will help establish a good population of good bacteria in a foal's gut. Nature's probiotic is fresh poop from a mature, fat, healthy horse that has no parasites. The worst thing you can do is clean a stall or pen so good that there is none available for a foal to eat. You can use Probios or another commercial probiotic, but fresh poop is better.
I have raised foals for about 45 years now, and I have treated a lot of really sick foals and have never seen permanent scarring. They can get a blistered butt and the hair will grow in a different color at first, but by the end of the summer, you could not tell it from the rest of the hair.