Perhaps you can tell this other blokes insurance company that the damage caused by their clients driving as compromised the optical performance of the headlight. As such their repair of buffing down the lens with the scratch still visible has not restored the headlight to its pre-accident condition & thus optical performance. A cracked tab, that has been glued back together is still a tab securing in place what it is supposed to, causing no negative effect on optical performance or vehicle legality, but their poor repair of the headlight may cause the car to fail an inspection. No insurance company can be allowed to hand back a repaired damaged vehicle in a non-road legal condition. The sole purpose of a headlight is for just that purpose, projecting light onto the road surface. If they come back with, the glued tab is not how it left the factory, well neither is a headlight with a single stone chip technically or a peeling decal. Where do they draw the line on what is factory specification / condition? If they say, well stone chips are fair ware and tare, then the tab crack was due to UV degradation of the plastic causing it to be come brittle (ware and tare). As an alternative, if your insurance policy has legal aid cover, you should inquire about using that cover to claim back un-insured losses from his insurance company that would cover the cost of you purchasing a new headlight. That way your insurance company is not getting into a battle about what should be replaced on the car as you have previously stated that they do not want to know.
Best of luck.