At Hull they should be able to get a 60 day tourist visa. Alternatively they will enter 30 days visa exempt, and you could take them to your local immigration office for a 30 day extension.
Depending who you fly with for your onward flight, you might be able to check your luggage through. You can then go through a transit immigration point that is usually very quick.
We fly here regularly, from the UK, with a return ticket for several months in advance. They always ask about it. We just say there is an annual visa in the passports, which they just glance at, and all is fine. Had the same thing with a one way ticket from Penang a couple of weeks ago. No problem.
If he is working somewhere else, might be worth getting a letter from his employer to say that he works month on/month off or whatever the arrangements are to explain his "commuting".
They are not telling you to go home. You can choose to not enter Thailand and go somewhere else. They can say "if you want to enter Thailand, you must meet these conditions", which might include a ticket home.