I was ill as a teenager/young adult. My advice would be to emphasize the compact part of "compact and accelerate". Don't waste your child's energy by asking them to invest effort in something that doesn't actively create value. But don't constrain their education or force them to learn less or think less deeply than they want to. I took almost double the number of courses most of my classmates took, but skipped a lot of classes, relying on reading (skimming) on my own to keep up with the course content. My education was structured in a way that meant only the final exams mattered, so I didn't have to deal with a heavy regular assignment workload that counted toward my final grades.
For subjects like PE I had an agreement (doctor's note) that allowed me to participate at whatever level I felt capable, which often wasn't much. In retrospect some regular but low intensity exercise/conditioning probably would have helped me maintain and develop stamina - eg yoga, Pilates, non-competitive swimming, etc.
Finally, as mentioned in the article linked above, the social aspect is really important for teenagers, and often overlooked. Being ill or fatigued can be really isolating. As parents, make a point of hosting [a few of] your child's friends for relaxing social occasions. E.g. sharing a meal, watching a (dvd/streamed) movie, video or board games, listening to music, just hanging out and talking. As a bonus you will get to know your child and their friends better. Also seek opportunities where your child can go out and be near their peers and hang out near the action without too many demands being placed directly on them - e.g. watching school sports, participating in drama (where level of involvement can be scaled from behind the scenes support to starring), etc. Make sure they are able to rest or leave when they've had enough so that they are not overtaxed. Socially unavailable teens can be seen as being standoffish and it can be a very isolating downward spiral. Friends who are engaged are a big source of moral and practical support for sick teens.
Good luck figuring out what's going on, and managing through this illness.
