I'm not sure if anyone who's non British resident but a legal visitor has applied in UK. It's the use of the word "residency" that seems to be the issue. Right to enter and stay in UK on a visa might be considered "residing" but it's a bit vague.
why doesn't he apply from where he is? If you think his proof of residing in UK is going to be an issue he can go to Thailand on 60 day visa exempt entry, then pop over to another country to apply. One where proof of being in the country is an entry stamp and hotel booking.
no. You enter the country legally. Some need visa some don't. Surely you know how he came to be in UK? The thai embassy in every country set the criteria for applying.
You have to have evidence if living/staying in the country you apply from. So if he's applying in UK, and he's not British, he will have proof he's there legally and some sort of address.