He arrived in Cambodia in Mid March; that of course does not indicate the date he first arrived in Thailand. Additionally it does not indicate whether the couple had a return ticket back to the UK; that said nothing to suggest they did not.
As I understand it, passengers transiting through Bangkok airport do not require further documentation if the flight they enter BKK on lands within 12 hours of the departure of their ongoing flight. It looks as though they intended to enter Thailand and for this a certificate that they have been tested for the virus with a negative result and a fairly precisely form of medical insurance are both required.
I also suspect that they flew to Cambodia on a sepparate ticket to their main flight into Asia from UK. Leaving Thailand during this crisis is risky. There was a story that did the rounds a couple of weeks back when passenger(s) flew from Malisia into Bangkok and were refused entry. On arrival back at Malasia they were refused entry there too,
I have only been back in UK for less than a week. I was rescheduled by my airline onto another airpline. Whilst contacting the airline was difficult it was possible by a chat line and once through it took less than two minutes for the operator to re schedule my flight.
There are flights available from Pnomh Penh to LHR, mainly Korean Air over the next two weeks, but of course if booking a new one way flight there is a significant cost.
There is a lot of unknows about Nick and his flights but whilst I have sympathy for him I think he has not acted wisely.