Seconding this. Always better to try and rectify any issues when you're on the phone, at home, without any undue time pressure while standing at the airport. Unfortunately, sometimes the DL/KL codeshare just doesn't cooperate when it comes to checking in anywhere but the ticket counter (frustrating, indeed). One thing to check - if you go to the reservation on DL.com and click on "view receipt," does everything match? Flight numbers, class of service, etc.? If there have been any schedule changes, upgrades, etc. since booking, and the ticket wasn't reissued to reflect that, then it will prevent you from being able to check in. If you do have a discrepancy there, it'll be way less stressful to handle it ahead of time.

If that looks good and DL is telling you they don't see anything, then it just might in fact be bad luck and she'll have to go to the counter.