Do you know who owns the dog? If so I'd send them a bill for the value of the foal and any vet treatment/pts. And if payment isn't forthcoming, I'd go to Small Claims Court. I'd also be making sure that the Dog Warden and police know about the incident, even if you are not sure who the dog belongs to.
I feel so sorry for your friend and the mare, what a dreadful thing to happen.