The payer is the owner of the account? I doubt that is AWS' default stance, because their contract is with the account holder, not the payer.

Me paying your bill doesn't give me ownership of your stuff - as far as AWS is concerned, your bill is paid, and that's the extent of their involvement, everything else is between you and me.

If what he writes is true, he remained the account holder and even had a backup billing method in place - something he probably wouldn't have if he wasn't the account holder.

I don't know if he's completely honest about the story, but "somebody else paid, so we decided they are now the owner" isn't how that works.