if she has thai citizenship would she have a problem with overstay in thailand anyway ?

she could leave thailand in may on her thai passport and show her u.k passport in uk and she should be allowed in.

only problem is she would be classed as on overstay with her u.k passport in thailand but there's no need for her to use u.k passport in thailand in the future.

maybe you should contact uk embassy and ask there advice .

I'm thinking that this whole situation is strange. A Thai person enters Thailand on their British passport. Why the #(*$% would they do that? Enter with your Thai passport and you don't have to pay for a visa or worry about your exit date. Additionally, this Thai citizen surely speaks and reads Thai, right, so they are automatically and immediately able to find countless official and extensive sources of information that are going to be vastly superior to any opinion you might find here, in English.

There are a whole range of reasons why Thai citizens enter Thailand on a foreign passport. Sometimes arrogance, sometimes ignorance, sometimes fear (the mistaken belief that they aren't supposed to be using it anymore). Sometimes they think that the expired Thai passport is no good to enter on (it isn't). And finally, a junior official refuses to let them in on a new passport which they obtained overseas.

The result is the same however. Despite being a Thai citizen, being stamped in on your foreign PP means you are subject to immigration rules, and as recent threads have shown, you will be forced to leave on your foreign PP as the immigration computers appear to be pretty good at cross matching data, which shouldn't be a surprise to anyone in this day and age.

Arkady has given the correct answer.

When re-entering and you have a new Thai passport (which was never stamped out of Thailand), ideally have your old passport with you which has the evidence of the last time you left the country. The immigration officer will stamp you in on either the old or new one. Both have happened to me.

Others, including many in my family (all dual nationals) have simply presented their new virgin Thai passport and have been stamped in with no fuss (in the case of my younger sister having not travelled to Thailand for more than 30 years on a Thai passport).

When none of the above works, others on this board, also including myself, have simply asked to speak to a superior who inevitably overides the junior stamper and let's you in on the Thai passport.

this is what arkady suggested :

When I flew from Bangkok to Luang Prabang I had no problem in swapping passports. It seems best for her to fly to a neighbouring country and return on the Thai passport.

but she currently has less than 6 months on her u.k passport so how can she fly anywhere except the u.k ?

maybe she could book a flight with her thai passport details then show the u.k one to the immigration at bkk then try to enter laos using the thai one ?

is that the way you did it arcady ?

why are the land crossing policy different to the air crossings ?

also is there no official legal way she can do things ?

Why do so many dual nationals choose to come into Thailand on their foreign passports? It's totally illogical, and only leads to problems such as this one.

Well for many years my wife entered and exited on a US passport as they said her old one was invalid. We eventually got that fixed as someone at Amphur years ago marked her out in Red = you are dead. Once we got that corrected (a fun day) and a new ID card, suddenly she could get a new Thai passport.

Hong Kong does not require six month validity of passport for entry.

Hong Kong (SAR China) (HK) Passport required. - Passport and/or passport replacing travel documents must be valid for at least one month beyond the period of the intended stay.

http://www.delta.com/planning_reservations/plan_flight/international_travel_information/visa_passport_information/index.jsp