A friend of mine lives here on extensions based on retirement, he is currently back in the UK for medical tests, before he left Thailand he got a re entry permit

Whilst he is in the UK he wants to apply for a new passport, when he returns to Thailand will they just transfer the extension over to his new Passport at the Airport? He will bring both Passports back with him

Also,

It is my understanding (I have never used one) that when you depart Thailand you have to put a date you will return to Thailand on the re entry permit, he is going to be over this date due to more medical tests needed, is this going to be a problem for him?

Date on re-entry permit has no meaning - will not be a problem.

He will need to show both passports on entry and later have extension of stay stamp transferred into the new passport at normal immigration office (that issued the extension).

His Re Entry Permit will be valid until his current permission to stay expires.

He will show both passports when he returns.

He will then report to his local Immigration office to transfer the extension.

Thankyou to you both