His options are, like they say, either go home and apply for another visa or go to another country where he can get a visa on arrival that also has a Thai embassy where he can apply for a new Thai tourist visa.
Fanny Gaijin That would explain it. Being from Croatia, he has to get a visa in advance of traveling to Thailand. There is no option for visa on arrival for people from Croatia.
Shouldn't take too long in Chiang Mai. Bangkok takes like 3-4 hours. Chiang Mai will be less. Need: TM.7 form (they will give you this at the office), one passport photo. Photocopies of (1 each): Passport picture page, current visa, entry stamp, TM.6 departure card. 1900 THB. Get all the photocopies before you go. Not sure about Chiang Mai but take a pen because they won't let you borrow one at least not in Bangkok.