Why did you friend assume it was the Covid and not some other issue? How did the doctor help him?

That 400 million number is BS though. Here is the methodology:

> After acute SARS-CoV-2 infection, an estimated 400 million people globally and 17 million, or 6.8% of Americans, based on Household Pulse Survey, experienced Long COVID symptoms, such as fatigue, cognitive impairment, body pain, headache, dizziness, sleep disturbance and a myriad of other symptoms and signs spanning multiple body systems [1, 2]

They surveyed people after the biggest societal disruption in 50 years and found some of them were tired.

This is why people don’t take long COVID seriously. It’s just a catch-all for feeling off.