I think CLT is just going to get rain and mainly just today, so hopefully the flight should still go out. The biggest issue I see is for potential for there not to be a plane available, because it is stuck in DCA or PHL.

Have them get there early. If there are other flights available to CLT that leave earlier tomorrow. You can see about using the travel advisory to their advantage and get them confirmed on that. If no seats are available, still have them try to standby.

If an earlier flight won't work they should be proactive and ask the GA to back them up on a later flight.