The important thing to prove is that she has good reason to return to Thailand, such as own property, has children in Thailand or any strong ties, has she had previous visas? Whoever is sponsoring her needs to show funds, photos of where she will be staying etc, the application has a checklist of enclosed documents, do a portfolio wth as much info as you can. Good luck.
Thanks for that Henry, current visa expires end of March, was looking at coming back to U.K. mid March, so would need to apply for visa before existing one expires, just bad timing I guess.
There is a receipt to say you've filed a TM30 which goes in passport of visitor, so if any visitor were to be passport checked for any reason there would be fines for both the visitor and the house master. And how long will it be before the departure card is cross referenced with TM30 reporting. I take the point, if there is no reason to go to immigration then you are "flying below the radar" so to speak.
But seeing how TM30 reporting is a counter terrorism measure it would only work if all terrorists applied for an extension of stay !