I would have to agree that he is not ready. I have 2 boys and 1 girl. They were all different. My second son, cloth diapered-showed signs at just over 2, was successful for a couple weeks or so, but then started having accidents. I was getting tired of the clean up and it was getting cold outside, I decided to try again after winter and he was completely ready and trained very quickly, but he was almost 3. But since he isn't yours, then you can not really know what is happening when he isn't in your home. Does he use diapers at home or does his mom clean up his messes?
Can she afford some cloth diapers? He may like them less and be more interested in the potty over time.