Hi Vikie
The law states that you must have hypo awarness to maintain your licence if you go here
www.dft.gov.uk/dvla/medical/ataglance.aspx and download at a glance guide, chapter 3 deals with diabetics.. this is the latest guidelines 2011..
As this is an hypo that happen durning his sleep it is different to a hypo durning the day, if durning the day (while awake) and he's able to pick up his hypo symptoms before his levels get too low (ideal upper ranges of 3) then he should be able to keep on driving,,
Just remind him to ensure his own safety etc, that he does need to test every time he gets on his mopped to ensure that his levels are ok, if he hasn't eaten for a while and feels that his levels might drop, then have a snack (I tend to have a Jelly Baby or two, just enough to ensure bg doesn't drop) and if he's having regular hypo's either day time or night, then look at his data to see if he's needs to make adjustments, if he's unsure discuss with his team etc...