If you really have a visa from outside Thailand, it just expires. If you obtained an in-country extension, you'll have to enter on a new visa and start the extension process all over again.
Any travel insurance is likely to be void in light of the latest UK government advice as could other insurances unless they cover you as a resident in Thailand. No medical certificate is needed as of today, 17th March.
The handling agents for KLM in Glasgow are particular Nasties with regard to the "30 day" issue. I was almost denied boarding once until I pointed out that my trip WAS exactly 30 days as I didn't arrive until "tomorrow"
There could be an issue at the airport of departure if his return flight is not within the 15 day window of his VOA, it all depends on the check in staff, some are worse than others.