If they are in Bangkok, they can self file easily (See
@Tod ********
post dated July 13, 2019).
If they are NOT in Bangkok, they can ask their local immigration office how to self file, OR, if they will only spend a few nights at the air bnb, simply ask the hotel they would stay at after they stay at the air bnb to print out their TM30 verification before they go to immigration to get the 30 day extension.
To my knowledge, immigration does not do random checks on people to see if they have filed a TM30. They only fine people if they go to immigration and try to get something done.