Are you kidding? Thailand is as tight as a fish's arsehole regarding pain management and anxiety/depression/sleep medication. Cambodia is a totally different matter.
Thanks so much to everyone that responded. Really helpful responses from everyone which put my mind at rest. I really appreciate the advice and information. 😀
My current thinking is that he book it himself. My mate is a very unworldly person from a poor background. I rescued him off the streets when I lived in Thailand 30 years back and have since helped support him and his family. He is more like family than a friend. It's not a surprise - he is aware of the plans. I am just trying to make the process easy for him. He would be flying into Don Meuang as it's close to home.
He has no passport. I was assuming an ID card would be fine for domestic flights - that would all be in Thai. And being an anxious, glass half full person, I always overthink things.