Originally Posted by
awpeters
I personally don't think the risk in this situation (invalidating 4 DFW-SFOs, GA trouble, future AA trouble as this could red-flag you much more than just a one-time "forgetting to take last segment", possible airport security/police getting called, etc) is worth the $150 change fee of changing her DCA-DFW and linking to your PNR to avoid UM fees.
I'm not sure whether this would raise a red flag at boarding. I'm sure not convinced by your claim it could rise to the level of getting airport security and police involved.
OP could give the GA a heads up at DFW--she won't be able to make the flight and is being picked up by someone else, just so they don't think there's a 12-year-old lost or abandoned at the airport and they could release the seat.