I have some suggestions that may help.
There is a neurological hospital connected to Chiang Mai University Medical School. It is on Suthep Road
https://www.cmneuro.go.th/EN/index.php
For day to day medical needs, the Geriatric Hospital can be useful
www.med.cmu.ac.th/en/service/excellence-center/gmc/
If dementia in some form is diagnosed, then residential care may be needed. Dok Kaew Gardens is attached to McKean Hospital, and is reasonably priced. There are other, less affordable privately owned places.
https://www.mckean.or.th/dok-kaew-gardens/
Somebody, somewhere, should be given a power of attorney to manage your friend's affairs. You could perhaps discuss this with him while he is still legally able to make such decisions.
Good luck to him and to you.