I have never ever been stopped at Thai immigration concerning having a one way ticket however I was once stopped by my airline at Heathrow (Eva airlines) and they were about to refuse me boarding, luckily I had my work permit with me and they let me travel!
Why does everyone just assume you can just fly here and start teaching? Does he have a degree in English education? I doubt it, how old is he? You can't just walk into a school and announce that you're a teacher and start work!
I've just written a letter addressed to the Thai embassy stating that if I die here my partner has the right to cremate me here and dispose of my ashes however he sees fit. It's enough.
Years ago yes but not anymore I can assure you, we lost some good teachers because of this. If your degree has education on it such as bachelor sports education that is fine otherwise no.