but this means nothing! They change the rules every other week, and even when they have a hard and fast rule it depends what the official in front of you thinks, if he/she is having a bad day - so are you!
this is bit of a grey area, I have seen it said on many of these forums that the onward travel must be to a country that is not adjoining Thailand, but nothing is sure about the crazy immigration system here, even the officials and the immigration office will give different advice on different days...
People might have problems from their airline if they don't have an 'onward' ticket out of Thailand, doesn't have to be a return and the rule isn't often even enforced, the airline can be liable to a fine if they knowingly allowed a person to fly into Thailand without seeing proof of onward travel.
Thai immigration website? Good luck with that, he will find what he thinks they have said may well be completely different when he is standing in front of the immigration officer...