I would have done exactly same thing! and actually i did a few times, one of those times i took pictures on how it was kept and reported it to spca. It wasn't a frog though and it didn't make it, but i tried. You did a right thing and I'm so happy that there are people like you who care.
if you know the owner have you tried to explain husbandry? just in case he would have another frog for sale.
You got an excellent advice already, your frog will be fine, i would however keep an eye on possible infection if his skin was damaged and bleeding when the guy was trying to peel the moss off him. I would also do a pedyalite bath for him and reptiaid to help with minerals that he is probably lacking due to improper care.
on the other note, he is totally gorgeous, little skinny, but with proper care he should be round and happy pretty fast!