I am not so sure, it might be legal but there might be problems later, because if she switches between these Passports it might happen that she has the Entry Stamp in one Passport and the Exit Stamp in another, this way it might be difficult for the Customs to see if she has left the Country or not, might be problems with overstaying etc. But as I said not so sure about it, but for my understanding it sounds odd doing so. Anyway isn't dual citizenship forbidden, I would think the moment she gets the British Citizenship she has to give away her Thai Passport???