My friends are in Thailand on retirement visa. They have a son (17 years old) in US. Can they apply for their son to live with them in Thailand? If so, how?

Yes, he can apply for a non-O visa in the US and get an extension of stay from immigration in Thailand (provided he lives with his parents, but only till the age of 20 (The age of majority).

Once he turns 20 he can no longer be a dependent and needs his own reason to stay in Thailand.

He can apply for a non-O visa in the US based on going to live with his parents and proof of parents living in Thailand on an extension of stay (copy of their passport with extension of stay) and maybe his birth certificate to show they are his parents.

He can get an extension of stay as their dependent until the age of 20.

He should first get a single entry non-o visa before traveling. He would need a copy of passport photo, page and extension stamp page of the parent who has the retirement extension. Plus birth certificate to prove relationship. Cost is $40.