The only other thing I'd suggest is getting paid for your hard work. It's obvious the neighbour either can't or won't raise 'em properly. He may even depend on the kindness of others; whether consciously taking advantage or not.
"Probably wouldn't be treating if it wasn't for the fact that we are so close together, and I don't want it spreading."
This kinda shows you're not so much treating out of kindness as you are fear (although you are incredibly kind to offer to help anyways). Fear shouldn't be a motivating factor to get along w/ neighbours.
I'd def expect something in return, maybe a chunk of goat or lambs meat at his next cull, some of his veggies or whatever. But nothing chicken-related, he may try to repay you w/ some of his sickly chickens. >.<
We are friends, And I honestly don't think he even knows there's anything wrong with his birds. He definitely isn't the kind to take advantage of people, or depend on them, he's always worked hard for what he has, and continues to do so, The odd part are some in a different pen are perfectly healthy. I was contemplating treating them for my own birds safety, and then I saw them, and I felt so bad for them, and I didn't want him losing anymore birds, that I decided to treat them. I have the stuff on hand anyway, so it won't be much of a problem, I'm the kind that wants to fix things, and these birds need fixing! lol. I'm probably getting a rabbit out of this whole thing, because they know I take care of all my animals, and they don't want it anymore.