Can a disabled foreigner obtain a power of attorney for retirement visa matters in Thailand?

A friend of the user, who had a stroke and is now in an assisted living facility, seeks advice on whether he can obtain a letter from his doctor to allow an agent, lawyer, or friend to handle his annual retirement visa without his presence at immigration. The discussion revolves around the possibilities of using a medical certificate, the requirements for setting up a power of attorney, and how to keep the retirement visa active despite being disabled.