I have opened bank accounts on tourist visas in 3 provinces, plus some more while living in Thailand.10 years ago you could easily open a bank account almost anywhere without breaking a sweat.
There must be some agents on Koh Samui in for example Chaweng or Nathon. Hop on your bike and ask them in person. The tricky bit will be legalization and translation.
He could have opened a Thai bank account way back. I opened my first bank accounts in Thailand on tourist visas.I had my 800k in the bank way before I applied for my first 1 year extension.