If he had a DC fault on his own boat there could be a problem. If there was a fault on the other boat leaking current (either AC or DC) there could be an issue as well.
Current thinking is against bonding of through hulls. They are composed of one metal and if isolated there is virtually no risk to them. If they are bonded they will become part of any "battery" created by a fault. A fault could be as simple as a bilge pump connector not sealed properly and in water or a faulty float switch.
Living aboard in Victoria Harbour