My friend currently has a ninety day stay with a non-o guardian visa, based on being a parent of a student attending a school in Bangkok.

 

Could you please tell me the documents he will need for a 1-year extension of this visa??

Proof of at least 500k baht in Thai bank for 30 days for the first extension and then after that it will be 90 days. That would be a bank book (and copies) to prove it has been in bank for 30 days and a letter from the bank confirming the account and the balance on the day it is written

Proof of his child having a extension of stay or non-ed visa based upon attending a school. A letter from the school stating he is attending school.

Copies of the child's passport photo page, visa and extension stamp. Their birth certificate to prove he is their father.

Copies of his passport photo page, visa, entry stamp and TM6 departure card.

There may be more required than what I wrote. Best to check with the immigration office where he will be applying to confirm what they want.