Another solution is that when it's time to check-in for the flight and print boarding passes, do not checkmark the one passenger.
If the rest of you board without checking that person in, Southwest will not page the person or hold up boarding because they never checked in. That person will have ticketless travel funds for that one-way originating flight available within 24hrs of the flight they missed. Those funds will be in their name only, unless it was booked as full fare or business select in which case they are refundable.
You could also ask the gate agent to remove them from the flight to sell their seat to a standby/etc, however I don't believe the gate agent can truly delete the reservation (just a boarding pass) without it affecting the rest of the group on the same PNR, or affecting that person's return flight.