I had it back in early March in NYC. At the time, everyone was discouraged from seeking medical attention unless they were on death's door, so I never got any. No idea what my blood oxygen levels were. I felt pretty crappy, though. I'm pretty sure I had pneumonia at some point because my lung crackled when I breathed (but I luckily never felt short of breath).
Hopefully your relative will do okay. The medical community has learned a lot since March, and has a better idea of the kind of supportive care that really helps. I can't speak to the specifics, but I have heard that it is common for things to get worse the second week and that it doesn't necessarily mean bad news.
Once he is on the road to recovery, he should take it easy. I was training for a half marathon when I got sick and tried to pick it back up again when I got better. Mistake. I definitely did not bounce back the way I do after a regular cold, and I think I made things worse pushing too much too fast.