It probably can only be fixed if he has boot2 bootmii or a nand backup to reprogram the nand. You can test savemiifrii, if savemiifrii is working you can fix it. Unfortunately savemiifrii probably is not working, failed priiloader install typically ends in full brick.
You can fix it with boot2 bootmii
You can fix it with nand progamming if you have a nand backup or the wii is very old
You can fix it if savemiifrii is working.