It was a used axle assembly. Meaning, it was used. Parts wear over time, thus becoming used. When a used part is, pardon the pun, used then there is a much higher chance of failure than one that is new or rebuilt. Therefor, only a 6 month warranty was given because they felt that the potential for problems with this used axle is greatly increased after another 6 months of use on a used axle.
He got 16 months on an axle that was used. Sorry, they only warrantied it for that 6 month period.
Ford cant see what happened to that axle 16 months prior, that guy is telling you a story or the stealer is trying to make you look bad.
So, did you charge him more than the axle could be rebuilt for?