I would simply change his booking - this will have no impact on the other bookings

6000 + 5000 to make the change = 11000 if no refund of points difference is provided to change - no additional tax to pay - so total cost 11000 + $30.69
To cancel and rebook would have a 6000 penalty , so no points refund, but would get the taxes back - so total cost = 11,700 points + $30.69

vs no show and booking anew
6000 + 5700 + $30.69 + $30.69 = 11700 points plus $61.38 which seems to be worst value

Cheapest option seems to be to just change existing booking