Can my friend work on a Non-O visa after transitioning from a B visa in Thailand?

i would bet he's not on ANY visa at all and instead on a yearly extension based on employment.

IF that's the case he doesn't need to switch to any Non-O visa either.

All he's doing is changing the reason for his extension from one based on employment to one based on marriage to a thai.

He'd get documents from the MOL showing he cancelled his work permit, documents from his employer stating he stops work on xx date, go to the immigration office cancel his current extension and then immediately apply for a new yearly extension based on marriage to a thai by meeting the financial requirements and turning in the correct documents. He'll pay 1900baht get a 30 day under consideration stamp and then when that runs down he goes back and gets the year extension inked in

He CAN legally hold a work permit on a year extension based on marriage, so IF he finds another job that'd be between him, them and the MOL to get it issued