You have to cancel the Extension of Stay, you can not let it expire.
You have to visit the Ayatthaya Immigration somewhere between 15 and 30 April with a letter from your employer and the cancelation documents from your Work Permit to cancel the Extension of Stay dated 30 April.
I do not talk about contracts, You have a stamp in your passport allowing you to stay as long as you work for the employer who provided documents to Immigration. That stamp has to be cancelled, dated with the last day of employement with that employer.
If he is on an Extension of Stay based on being employed this has to be cancelled at the Immigration Office on his last day of work. This can be done in advance, he has to apply for a new Extension of Stay, this could be with reason married to a Thai National but he has to meet the requirements otherwise he will get a stamp that application is not approved and 7 days to leave the country.
There are total different rules and regulations between being in the country using a Non Immigrant B visa or using the Extension of Stay based on employement.
I would say get this clear before the wrong advice is given.