I asked him to run "nmcli dev wifi", which worked fine and showed all the available WiFi networks, confirming that the WiFi hardware was working fine and network manager properly working (and that Cockpit WiFi manager must be using some other command to do a scan, which wasn't working).
He then tried connecting to a network on the CLI by running "sudo nmcli dev wifi connect [YOURSSID] password [YOURPASS]" and that returned an error message about a security key being missing. I googled the error message and some fixes came up - https://www.google.com/search?q=nmcli+wifi+connect+wireless-security+key-mgmt+property+is+missing - the customer tried those (resetting some nmcli cfgs) and that fixed everything and Cockpit WiFi manager was then working great.