Is he a fatty like me? My Zwift-FTP was ~320 (couldn't hold that for an hour, but could do ~300) when I was 105 or so kg (and I was probably doing ~10-12 hours a week). I don't know how much of that was due to using a powertap hub that was 8-10 years old and may not have been accurate. Hub got trashed when I got hit, so, now that I have assioma duos, I can't ride with them both and compare. Not riding for 3 months, having a nagging back injury, and having gained 15 pounds (it was a ****** summer) means that hopping back on the new pedals when I get my new bike built won't be apples to apples comparison.
Honestly, though, miscalibrated powermeter sounds likely.