Probably "tincture of time," as one physician put it. And toward the time she recovered, she learned of a physician (neurologist/psychiatrist) who took a great deal of interest in working with CFS patients. She was, and still is, on a great many supplements. Perhaps they helped, perhaps not; I doubt that they did any harm. Little by little she got better. It was not a sudden recovery. She has been free from long term infections since then. She did get COVID in 2023, but it did not turn into Long-COVID.
The only way to keep from getting Long-COVID is to not get COVID.