This is an entirely normal failure mode for these pumps. They'll fail like this on a full tank or with 5 miles to go. Starved of fuel has nothing to do with it. Neither has whatever snake oil he wants to put in it, but alas you're in his hands now so its a case of nod and smile and pay the bill.
The D2 pump is unique as it has an extra outlet on the bottom which drives suction jets to balance the two sides of the tank. The variables are whether he fitted the correct VDO pump and whether he fitted it properly. VDO properly you'll be able to use it normally. The wrong pump, or the VDO incorrectly fitted, then it'll run out of fuel at 1/4 - 1/3 tank as the suction jet won't run so it will drain the right hand side while there is still fuel in the left hand side.
Its not unheard of for the suction jet gubbins in the tank to fail, but its extremely rare. Prestige have replaced a couple of tanks for this problem, and I have yet to look at Big Red which will randomly run to empty or run out at 55 miles remaining.
I always recommend to carry a gallon of fuel after a pump replacement just in case, so fingers crossed he put the VDO pump in properly and you don't need it
