Cheryl clearly has a caring and compassionate heart...unlike the 'breeder' from whence this filly came. Even a foal suspected to be lethal white deserves the best start it can have; close observation(as opposed to being out in the field with other horses!)might have insured that the foal got the early nursing it needed, for optimal strength. IF observation proved that the foal wasn't passing stool, and/or was becoming uncomfortable/distressed, then the kind thing to do would have been to euthanize--sounds like the 'breeder' had no intent to do any of these things....not to mention even taking the risk to start with, as Dona noted!
I would very much want to know for sure if the foal WAS a lethal-I would either take the last poster up on his offer(Animal Genetics), or, have the foal necropsied, which will also prove the situation.
20 or so years ago, I had a full-sized Overo mare give birth to a lethal(much less was known then; the sire appeared tobiano, turned out he was a tovero). It was a healthy-appearing, hearty colt foal-nursed and behaved normally, but NEVER passed stool. Born early in the AM, by late PM he was showing signs of distress, and I had him euthanized. This was on Easter Sunday-fortunately, the lady vet lived right next door to the house(my friend's)where the mare and foal were-but we just left the foal in with the mare, and the next AM, the vet came and necropsied the foal right there. His intestines ended in a tiny little 'tube' about the diameter of a soda straw.