Why has my client’s cat started pooping outside the litter box and how can we deter him from doing this in the future?

This is simply an idiosyncratic behavior that has crept in. It is, in a sense, a desire for novelty and not any sign of illness or a willful act to be a bother to the human occupants. Keep in mind this is wholly instinctive to begin with, not something that is trained to happen. It is entirely the animal’s doing and when they begin to vary from their routine, there is not much humans can do to deter this. The only course of action would be to be vigilant and remove the offending matter as quickly as possible to limit its reinforcement of the behavior.