I understand exactly how your friend is feeling. I have a chronic immune disease and I’m on a tough immunosuppressive that I’m constantly ill. Especially in winter I catch colds and pneumonia all throughout the season. The university always want a letter every time I’m off ill, my academic advisor is understanding but the hub say I need to hand in a letter from the doctors. And my consultant is happy to do it for free but he’s getting annoyed that every two/three month, he constantly has to write a letter which is practically the same as my illness hasn’t changed and it never will