What should my friend do if his recent entry into Thailand was incorrectly stamped with a tourist visa instead of a re-entry stamp?

Okay, this is how it is supposed to work.

It goes without saying that you people ALL need to check your entry stamps when you come into the country BEFORE you walk away from passport control.

Bad stamps are YOUR responsibility, meaning if you're stamped wrong YOU have to take the steps to get it corrected, and if you're stamped in for TOO many days you do not get to stay because immigrations stamped you wrong, 😕

IF you are stamped wrong at the airport going back there is the fastest way to correct a stamp. BUT you can also go to any immigration office to get it corrected as well

However that is NOT the case with LAND entries

IF you are stamped wrong on a land entry you have to go back to that border to get the stamp corrected. Immigration offices inside the country won't fix them

So one last time CHECK YOUR STAMP BEFORE YOU WALK AWAY FROM PASSPORT CONTROL.

Yes, definitely he needs to go to get the stamp corrected. Technically, he cannot work legally on a visa-waiver stamp (it's not a tourist visa entry nor a visa-exempt. it's a visa-waiver entry). Doesn't matter how long he has been already staying in Thailand.

The best is going to go to the entry point to get the stamp sorted, much faster and easier to do than at the immigration office.