Hi,
During my long experience...and varying degrees of control of my T1 diabetes, I have found that high BG is like anything else. If you are used to running high..say 13, you can actually tolerate it quite well as you are used toit. this doesn't mean thatit is not doing you lots of damage though...as it is. Now a days, my levels are much better, and if my BG creeps up to say 13, I would eb unlikely to drive, as like others have said, my eyes go fuzzy!
I guess, you could view it like driving if you had beeing drinking alcohol. A seasoned drinker, would say, thatthey were fine after a couple of glasses of wine, where as an occasional drinker ike me, would not even get behind the wheel after taking a sip...as I know the effects would make me unsafe. Unlike alcohol though, I am not sure that there is a limit...I think there should be. I guess when they worked out teh safe to drive alcohol limit they could look at millions of pieces of data, and asses the variations and confidence in the data. Not sure that data exists for hyperglycemia.
Agree with Nigel, your Dad needs to conact his medical team and discuss.