With the comments of the white patches and red undercoating, it almost sounds like he possibly has thrush, which is a yeast infection of the mouth. That could also explain the burning sensation. He can see a doctor or a dentist to confirm the diagnosis, and at least it is cureable.
I tyhink you need to find out why there is this lack of saliva flow. I do not know how old your friend is but it is obviously not normal to have to wake up in the middle of the night to wet your whistle so to speak. I would suggest that you see an oral surgeon for a diagnosis.