I agree that you should do something and that perhaps having a word with his GP and contacting the DVLA directly would be a good idea.
Like you say, Mosschops, you wouldn't let this person transport your DD around. If the person is a genuine risk to ther drivers then they must be reported, for their own good as well as that of the general public. I know that it might seem harsh, but the alternative could well prove fatal
I would do it, even if it was my DH, and I would hope that he would do the same if it were me. My motoring independence is NOT worth a life.