ac500u,
I've stuck a few displaced thresholds in during my time. Before I come out with my version of the warnings that j-t and mutt were making, let me mention the ICAO section that goes to the heart of the aircraft operation and is relevant to your request: Annex 6 — Operation of Aircraft Part 1 (8th edition):
5.2.11 Landing. The aeroplane shall, at the aerodrome of intended landing and at any alternate aerodrome, after clearing all obstacles in the approach path by a safe margin, be able to land, with assurance that it can come to a stop or, for a seaplane, to a satisfactorily low speed, within the landing distance available. Allowance shall be made for expected variations in the approach and landing techniques, if such allowance has not been made in the scheduling of performance data.
There is nothing else in Annex 6 that makes landing short illegal or legal. There are also examples in the Attachments of that Annex on how the aircraft manufacturers calculate landing distance, but while informative to read, they don’t dictate how the pilot must land the aircraft. They are presumably for standardisation and the avoidance of over-optimistic performance numbers such as you mentioned earlier.
OzExpat put a nice argument in his post above, which suggests your lawyers can take forward this short, but powerful section 5.2.11 to build the case that the flight was legal because you landed safely etc etc. The pavement strength issue is a rarity and not normally expected to crop up in displaced thresholds (but I'm not ruling it out). If the local groundstaff can drive their truck up the runway or drive the firetender up it, then it will support a light aircraft.
Now back to the warnings. I find the calculation of displaced thresholds are not easy and are somewhat counter-intuitive. We always have 3 or 4 people do the calculations, and after going around a bit, they hopefully come up with the right answer. That takes a few days to work out, and I wonder how your friend can do it so quickly while landing the aircraft. I'm sure he is aware of the obvious traps, such as:
It is easy to land short once and get away with it, but the principle behind the design of displaced thresholds is that you can land safely every time, and I still don’t see how your friend has built longevity into his operation.