Do they have open days? He could try and attend one, and ask his question 'informally'.
How long did he manage to stay in mental health? I've known a couple of nurses who resigned without having found a new job (including male nurses who were single, i.e. were the only breadwinner in their household) because it was such a hard job! I've worked in caring myself (basically, elderly people who can no longer live independently) and I would not do it again, even if it was well paid (which isn't the case)!
If he'd like to work with soldiers who have p.t.s.d., maybe he could look around for charities who do this type of work, or charities who help discharged soldiers find 'civilian' work, accommodation (difficult to get on the property ladder as often they have little or no credit history) etc. My father fought the Japs during WW2 and he had PTSD, though he never sought medical advice about it (he was a commanding officer, so was used to being 'in charge' and watching out for his men, not to being 'mollycoddled' himself). It didn't interfere with his life too much (apart from nightmares and flash-backs, which became more frequent as he advanced in age, after retirement). His own way of dealing with it was to try and forget the whole experience: the subjects of the POW camps was absolutely taboo. However, if he hadn't come home in the golden 50's, when there were jobs galore, which enabled him to launch into a successful career, I don't know whether it would have worked so well.