My Canada trip, I checked two bags, orange and black ones. The larger Carradry bag was my carry on bag and the smaller pannier was my personal item. See photo. I carefully measured everything first and everything fit the size criteria for the airline I was on.
An alternative is try to find a very lightweight duffle that matches the airline carry on size (airlines are not uniform for carry on sizes), and another bag that perfectly matches the personal item size. It is possible that all of her stuff could fit in those two bags.
This bag is slightly larger than most airline carry on size criteria, but of you do not pack it full, then you can shove it into a smaller volume if you have to. Just make sure that it does not have fragile stuff in it, if you are near an Ikea:
https://www.ikea.com/us/en/p/goersnygg-bag-blue-60499261/I usually wear my helmet onto the plane, if asked I tell the airline staff that I do not want a baggage handler to crack it, I put it in the overhead once on the plane. Only once has an airline employee commented, he said that modern planes are pretty safe, and he was smiling when he said that. I made my comment on baggage handlers, I got no argument back.
If airport security inspects anything she checks, it would have to be something that is easily put back together, if just a strap I would not trust security staff to strap them together well enough.
I use two luggage tags on anything I check. Second photo is one of my luggage tags when I got home from my last bike tour. If she straps two panniers together, put two luggage tags on each pannier.
I have not carried my Ortlieb panniers onto a plane but the Backroller would qualify as a carry on and the Frontroller would meet the personal item size criteria.