Have only read a few of these posts, but here are my thoughts.
My background: psychology degree, active interest in psychology and counselling. Lifelong Christian, in past was a member of charismatic churches (though never really 'experienced' the outward stuff such as tongues). Now my faith journey has taken me to a more 'open' outlook on my faith (sorry, that's quite vague - we'd need to chat for hours to explain!).
Specifically answering your question about tongues and mental illness, IME it is very common for certain church organisations to wittingly or unwittingly nurture a sort of 'oddness' about the way they encourage practice of faith. This can be as harmless as using lots of irritating jargon (people use terms like being 'under attack' or 'doing things in the flesh' - makes me want to say 'speak in English, please'). More 'extreme' examples are seen in many African churches where they practise exorcism or healing by physically beating the person to get the demons out .
As one wise African once said, "Jesus never beat anyone when he was practising healing, etc, etc, so why on earth do you think you need to?" In essence, if Christians take Jesus as their model of how to live life in all its richness, hilarity, sorrow and joy, there is no precedent for this sort of 'oddness'. He was totally involved with the everyday, ordinary people that he spent his time with - in fact, they were drawn to him. He did not shut himself off from society. He did not develop bizarre personality ticks and then put them down to super-spirituality. He was a normal bloke, as well as all the extraordinary stuff about him.
So IMO if your client is portraying bizarre behaviours in this way, it sounds as though he has absorbed something of the culture of the church organisation which he is a part of, and that is something which, in itself, is not a normal part of being a Christian.