I had something similar happen a few years ago. DL had reduced the frequency of the intl flight I had booked, so my flight was moved to a different day than I had originally booked. No big deal, I confirmed the changes on the phone and that was that.

Flying FLL-ATL-XXX, checked in at FLL no problems, checked bag, given boarding passes for each segment, same as usual. Flew first leg to ATL, dicked around in the Sky Club for awhile, went to board at a leisurely pace, well after boarding began. The boarding pass scanned and made an unhappy noise.

The GAs gathered around and typed away furiously at the computer, I explained that there should be no problems with the ticket, after all, I flew in from FLL without incident and had even checked a bag. One of them appeared to figure out what was wrong, told me I needed to go over to the service recovery desk and have them reissue and told me exactly what to tell them. We were getting dangerously close to takeoff, so I took off down the concourse and the desk was able to resolve it quickly. I managed to board in time and life went on as usual.

Never really clear to me what happened, but that day I did remember thinking how lucky I was that it happened in ATL where it could be resolved easily vs. BOM/ICN or some other distant outstation.

Hopefully DL takes care of this girl, as it sounds as if this was through no fault of her own...