A good all around supplement for internals (digestion, etc...) and externals (coat, skin, hooves..) is Equipride.
My filly is going gray, which was a little saddening at first because she is a beautiful grulla, but because of this I came to the realization that as long as she keeps her brain thinking on both sides and her overall sweet disposition, I don't care if she turns purple with yellow polkadots! You might want to have a converstation with the owners explaining that the graying process cannot be stopped nor slowed. If it's "the horse inside" they love, color outside shouldn't matter!
One is closer to God's heart on a horse than anywhere else on earth!