My wife and I got Covid during the first major wave (March/April 2020). I had no symptoms at all, but my wife ended up getting long Covid. GI issues, extreme tiredness, shortness of breath, brain fog, sleep disruption, etc etc. She lost maybe 10-12% of her body weight.
After maybe 8 months of this we somehow decided to do a low-Fodmap diet (I participated for support). That included cutting out gluten and dairy (except butter). She healed right up. When we were re-introducing foods, for whatever reason, mushrooms and garlic both had negative effects, and we ended up keeping both out of our diets for a couple years.
Anyway, that's all to say I'd recommend people seriously look at their diet and try to spend a month or two doing low-fodmap if they're chronically suffering from long Covid. Worst case it doesn't help.
So, I'm not doctor, but I know that a lot of autoimmune diseases (I have a couple) can appear after a bad infection that your body fights off. So getting checked for the common AI diseases (Hashimoto's/Grave's, T1D, Lupus (It's never lupus), Crohn's, etc.).
Anti-inflammatory diets seem to help a lot of people with autoimmune disorders, because a flareup typically stems from stress or inflammation. Removing dairy and wheat didn't do anything for me, though.
I never got COVID, but I had a bad flu when I was a teen in the early 00s, and "new" research suggests that is when my body flipped a switch and now I develop a bunch of antibodies that attack a few systems, but good news is I'm basically NEVER sick anymore (might just be a coincidence, though).
My fiance developed an intolerance to alliums (onion, garlic) and mushrooms around the start of COVID, though he had never tested positive for the virus. Low-Fodmap diet helped us to find the culprit as well.
Yet another anecdote, but my wife and I both got COVID. I was quite a bit sicker than she was.
I later found out that I'm a type II diabetic, and almost certainly was at the time I had COVID given the timing. I had fairly severe fatigue symptoms preceding that diagnosis, and the diet/exercise changes I made to bring that under control look a lot like what your wife did, and also seem to have alleviated the fatigue.
There was about a 12 month separation between recovering from COVID and my own fatigue symptoms. Had the timing been a bit different I likely would have assumed I was suffering from long COVID, and would probably have been less inclined to see a doctor as a result.
None of this is to suggest that any given person is suffering from diabetes, that long COVID should be treated the same way, or that any given person with long COVID is self diagnosing. For anyone that is though, definitely consider talking to a doctor. Coincidences happen and you could have something that is both unrelated and treatable going on.
After maybe 8 months of this we somehow decided to do a low-Fodmap diet (I participated for support). That included cutting out gluten and dairy (except butter). She healed right up. When we were re-introducing foods, for whatever reason, mushrooms and garlic both had negative effects, and we ended up keeping both out of our diets for a couple years.
Anyway, that's all to say I'd recommend people seriously look at their diet and try to spend a month or two doing low-fodmap if they're chronically suffering from long Covid. Worst case it doesn't help.