Hoof supplement won't help here. The soles are so soft from all the moisture, the vet figures the mule might have brusied itself,causing the abcess.
I talked to one of the vet at Red River Equine about hoof supplement to help harden the soles of an accessed foot and he said that it helped. I told I thought it did but a lot of people tell me I am full of it. I know I've seen a difference on the ones that I've tried it on.