Very sorry to hear of the loss of your friend. You give a good message: pay attention to your body and to your health period.
Before COVID was recognized or known of, for about a month I was fatigued like never before, exhausted, low energy, and I chalked it up to stress, low blood sugar, etc. I would run a fever for about ten minutes and then it would go away; this went on for weeks. From all the stories that I've heard, I think I may have had COVID.
It is a shame that your friend went to the hospital and the hospital may have waited to long to figure out what was going on; in light of this disease being highly contagious, you would think that they very first thing that they would have assumed was COVID.
I think a good thing for everybody to have on hand is an oxymeter; they are relatively inexpensive and even though people may feel that they are breathing ok, sometimes the oxygen saturation is actually low and that is the time to get to the hospital to make sure you don't have COVID.
Prayers up for your friend's family and for you as well.