As a UK national there are a few anomalies with motorcycles and travel insurance
If the families assertion re a 50cc maxima is to be believed then that a 'new to me' policy restriction but it likely refers to the UK full car licence which allows a sub 50cc 'moped'. So if he like most adults had a licence to drive a car, by default he would in the UK be OK for a moped too.
This leads to anomaly #1 in that UK car licences show the little motorcycle graphic (for 50cc mopeds) and it consequently also appears on the respective IDP. By virtue of this countless UK folks in Thailand are riding motorcycles (in excess of 50cc) with these documents. A ticking timebomb as come the day...no travel insurance cover!
Many UK travel insurances give absolutely NO cover for motorcycles
Some policies throw in cover for sub 125cc Motorcycles
Travel insurance with cover for > 125cc (which many newer Thai 'twist n gos' now are) is as rare as hens teeth - I know of only one.
None of the insurers (AFAIK) extend personal liability cover for riding motorcycles
So even if you are in the favoured position of having a policy with sub 125cc cover, and possessing the relevant UK motor bike 'rating' that covers that size cc / equipped yourself with an IDP, suitable 'sissy-lid' & found a 125cc rental - you still bear any liability incurred, and will fall foul of any alcohol or substance induced impairment clauses.