Can an American tourist extend their 30-day visa at Jomtien Beach Immigration?

Jomtien only allows you to apply for a tourist extension during the last week of the existing stay so make sure your friend doesn't go too early or else he'll wait in line and get sent away.

His arrival point is irrelevant. His local immigration office is where they get an extension. Some will require a TM30 for the extension. Some won’t. Without knowing the office that they’ll use we can’t advise what they’d need

IF he is in Pattaya, he needs to have a TM30 filed registering the Address or hotel where he is staying.

Jomtien Immigration office will accept the application for the 30 days extension of stay during the last week before his current STAMP expires.

The 30 days are ADDED to his current STAMP ending date.

These are the common found requirements for the 30 days extension of stay...

He can get a 30 day extension at his local immigration office