My thanks for the informative replies so far.
Whilst I'd read the CAA requirements, as a former engineer I'm well aware CAA documents can be open to interpretation. Hence I thought asking the question would provide more direct answers which has proven to be the case.
I was reasonably certain his condition would negate gaining a PPL however, if he declared it of course.
The hard part will be convincing him it's in everybody's best interests if he ceases flying training. Despite his condition, he actually holds a responsible job but as I say, the responsibly for flying an aircraft is entirely different as we know.