We have a free/spay neuter clinic locally.
The bad thing about the shelter, is the animal has to go through their intake process and regular adoption before they will help you out.
They don't really "foster" animals.
Pretty much if they take the animal in, they house it, (its exposed to the others in the shelter, including the fleas

and the diseases, as no shots or de wormers are given untill the after animals are adopted out)
So I'd be exposing the cat to whatever, a bout of flea infestation, (which I dealt with when I got MY cat) and I'd have to pay the 50.00 adoption fee after the 24hr or 10day waiting period depending on the shelter just so I could get a free/spay neuter certificate and have them give the cat its shots and worm it. According the owner, though it was given shots when they got it.
If it comes to that, I think I may skip the shelter altogether, and just pay the 36bucks to my vet for its shots/worming and get it neutered at the clinic.. lol
I'm hoping the fear of having to bail their cat out of the "clink" and facing unknown trouble for neglect of the wounds will encourage them to keep him inside where he's safe, and treat him with the medicine I gave them.