No way is this a "favor" for the OP to be put on the UG waitlist. This is what is supposed to happen. The SWU is valid for the entire single direction of the trip (assuming the SWU stopover rules are followed), and even though it was "used" on LAX-SFO, a rebooking due to mechanical (or even wx in this case) does not forfeit the downtrip use of the SWU. Period. The computer may think so, but the rules sure don't.

Now, if the OP doesn't clear, it would be technically correct to say that the SWU was used for the LAX-SFO trip and there's nothing to be done there after the trip in terms of refunding the SWU or swapping for a CR-1. However, given that the reason for the misconnect was UA's fault, it would be reasonable for United to refund the SWU to the sponsor as compensation. I'd definitely have the sponsor make that request to the 1k desk.

Couple of experiences: pre-UDU, was flying JFK-LAX-SFO, cleared all legs on a CR-1. JFK-LAX went fine. LAX-SFO canceled, rebooked on a later LAX-SFO, waitlisted for UG and ultimately cleared. No additional instruments were taken. Another time, same route, same situation, actually did standby with the GA on an earlier LAX-SFO, confirmed in first immediately, no additional instruments taken.