Long bus rides are a pain, but a cheap flight to Vietnam is fine, or elsewhere. Can one drive in and out? He has a car, registered in Thailand. Living near a land border is a convenient solution.
I know.I couldn't believe it when the airline crew said, "Yes, we are being serious." (my McEnroe moment). There wasn't any doubt that I'd be leaving Thailand... I had a job to go back to in UAE in mid-August. So annoying... they should inform people in advance about this. For sure that there will be some advice somewhere, but this was the first time I'd come across this rule.
Yes, you must have an onward flight booked from Thailand to another country within the 60-day period. Some UK airline check-in desk flight crew can be strict on this and not allow you to board from the UK to Thailand without it. It happened to me once in Manchester airport. So, I had to book a flight out of Thailand there and then at the airport using their computer. I just managed to board my flight in time (I had planned to book a flight later because I'd be in Thailand for 5 weeks).