First, I applaud you for trying to potty train a child! Especially when you've never done it before and you don't have children of your own. Second, although you think his poo smells really bad, you are not being judgmental. I have a 24 month old son and I think his poo smells really bad!! The older and older children get, the worse their poo smells. However, I can understand why the previous comment was mean-spirited. As mothers with children, sometimes we are offended by comments or actions or other women without children. We have to remember not to judge the other women because they don't have children so they don't know, but their comment/actions may still sting.
The problem seems, from the little information that I have, to be that his parents are apathetic to him being potty-trained. Perhaps they are because they are busy and don't have time to deal with it. Or perhaps they are because they've already established a bad habit of putting him in diapers and it's difficult to fix. Or perhaps they're relying on you to do all the work. Either way, they do need to get involved.
My son is 24 months old and he's potty-trained. However, we still use diapers when he goes to sleep and when we go out in public (Because when he has to go, he HAS to go. He can't wait until I park the car and find a bathroom).
Have you tried using the Pampers Pull-Ups? That may not be what they're called; it's the diaper that children can pull up and down like underwear.