That’s why I suggested he buy a return ticket to Thailand for his wife. Since it seems his wife has never visited the UK before, having a return flight will help increase the ‘credibility’ that she will return to Thailand
Getting a UK visa is very easy, but it depends on the purpose of your visit. I’m Thai, and I always get visas everywhere in the world, but my purpose is always genuine. My purpose is to travel for pleasure, and I have a return ticket. I have been to Europe more than 14 times