1 hour ago, bigt3116 said:

The OP's friend is going to use both the old and the new passport when he enters

Correct, 

I just explained why the immigration is sensitive regarding name change and passport. ????

7 hours ago, LikeItHot said:

I can't speak for him but I am getting dual citizenship and the new country will be using my birth name which is different from my current passport.  While it should be ok it would be better if everything matched.  Naturally I needed to think about Thailand before modifying my current nation passport and it's effect on my visa and Thai banking etc. After research I decided to deal with problems if they arrise rather than rock the boat prematurely.  Bottom line is he should have researched this thoroughly before making a change, not after but also it is not always an indication of being surreptitious.  One thing for sure is that Thailand will make it as difficult and expensive as possible for him.

Thanks for pointing out the coming problems with the banks.

I asked Bangkok Bank,  and the answer was: a Thai will have a certificate of the name change. Without such a certificate they don't know what to do. 

 

BTW I also called immigration hotline.  They answer the phone, on a Sunday afternoon!

They don't see any problem at all. Just enter with both passports,  get a certificate from your embassy in Thailand,  and then go to immigration to rectify your information and transfer the visa. Maybe it's really that easy? 

In the end, he got a new non-Imm O in his new passport (that wasn't easy, because his new name doesn't fit the format of the application template).

Entering Suvarnabhumi, he just showed the new passport.  No problems.