A Swedish couple I know have a 90 day non-immigrant 'O' visa. They are staying 98 days in Thailand. They're an elderly couple so don't really want to do a visa run unless absolutely necessary

I believe that a seven day extension is available at immigration. I also believe that immigration at the airport generally overlook a one day overstay, and in any case they would be happy to pay one day's worth of fines.

The question is can they get the seven day extension or will it be denied as they actually need eight days? Do immigration ask to see return flight tickets?

It would be Phuket Town Immigration Office.

They sometimes do require show of ticket but not sure how close they check them. It is not a real extension of stay but a disapproved request for extension that allows you to stay 7 days so more of a favor than a real product. Expect they will try to be helpful to an elderly couple if asked politely.