I don't see why she can't buy a SIM. Very little of the language on the phone comes from the SIM, if the operating system is in Thai, most of the parts of the phone will remain in Thai. When you go to Thailand you just buy a Thai SIM card and pop in in your phone right? Apart from the odd SMS from the network trying to sell you something in Thai language, you have no problems actually using your phone right? It will be the same for your girlfriend.
If by any chance the SIM manages to change the language settings to English - change them back to Thai manually. I bought my Samsung in Thailand and its language was set to Thai - I changed it to English without a problem. I have both Thai and UK SIM cards in it and neither affects the language.
You may have to set it up for her when the SIM is first put in and before that, make sure her contacts are in the phone memory not the SIM card. The biggest problem I see is her forgetting where her Thai SIM card is when she's ready to go back .
The alternative is Roaming on her Thai SIM and that will cost a fortune. In any case, if she's wanting to call home/friends, she will almost certainly use LINE (most of Thailand does it seems) and if you set it up for your home Wifi she won't have a problem calling anyone on LINE.