Chaing Mai Thailand has a bustling expat community, redonkulous low cost of living, and nationalized healthcare.
I traveled to Thailand on a work trip (I did a lot of international travel for a few years at my job) and then spent a couple weeks on vacation with my husband to check it out as a potential retirement spot since it sounded so good on the internet. My take away was that it is a great place to vacation--while I was there for work, I stayed in gorgeous hotels--, I would never ever want to live there. It seems that most people who end up successfully retiring in Thailand and being happy there are single men who get a Thai girlfriend or wife who helps them navigate the culture.
I visited in February (Bangkok, Chang Mai and Hua Hin) and the weather was extremely hot and humid. Outside of tourist hotels (which I'd advise only staying in for vacation and not for checking out the area for potential retirement) AC is very rare.
Since it was mentioned that your friend has health issues, I think it would be difficult for someone with health issues if they are mobility related since while there are lots of public transport vehicles, there still is a lot of walking required and dodging of mopeds with crazy drivers. Also, some of the transportation options will try to rip you off if you look like a tourist/Non-Thai. Handicapped accessible is not a thing in Thailand.
Looking non-Thai seemed to make my husband and I targets for a lot of potential scammers and/or sleazy salespeople.
We stayed in a local hotel/bnb type place in Chang Mai and the electricity hook up made me a little scared.
Driving is every person for themselves and pedestrians better watch out.
My Thai work colleagues were very polite--I was there to teach a class--but didn't really speak up to ask questions or interact. My American colleague who was on a 6 month rotation with his wife and toddler told me he hated it there--it was very difficult to get candid dialogue, feedback or problem solving. The culture is very different than what I was used to. In most of the countries I visited for work, outside of work when we went on work sponsored dinners or events, I was able to get to know some of my native work colleagues as people but this didn't really happen in Thailand--it felt like there was barrier.
Anyway, like with any location--I'd highly recommend that your friend visits --and not the tourist hotels--to make an assessment. Because Thailand does sound great just from reading internet info and blogs.