The correct command for lncli is (also referenced here):
~/umbrel/scripts/app compose lightning exec lnd lncli <command>
Has he tried reflashing the MicroSD card, or any other hardware changes were made with the SSD like the USB cable he’s using or anything? If you can run a new instance too of the Lightning Node app anywhere as well he can recover with his 24-word seed phrase. Happy to assist getting his Umbrel up and running again as well