I doubt it is anything specific to the G26 as a model. Probably more just this specific gun needs more break in.
If you can shoot it, but she can't, have you thought about maybe finding an instructor for her. Maybe the way she holds it can improve the issue.
For example, I'd wonder if she is using strong push out with web of shooting hand, and pull back with the support fingers. The shooting arm may want to have elbow locked, and shoulder locked. The support arm elbow may want to be bent, pulling the gun firmly into the web of her shooting hand (web between the thumb and index finger).
This ideally would provide stronger brace for the gun to recoil against, thus helping it to cycle reliably.
Also, suggest using American Eagle 124gr or Lawman 124gr, for practise, and something like Gold Dot 124gr for carry. Those rounds, especially the Gold Dots, with help the slide to cycle. You might already be using equally stout ammo, but if not, it is worth a switch.
With the grip I suggested above, she may also want to put her support side foot (left foot, if right handed) forward. This may help her to have more mass leaning into the gun a bit. I don't mean for her to be out of balance, but don't want her going backwards from the recoil. Her posture should be a bit forward. Thus helping to give the gun something firm to cycle against.
You, maybe because you are larger or stronger, or maybe more experienced shooting, might not have to think about these things, and might not have problems getting that G26 to cycle ok. But if anything about her shooting allows that gun frame to come back at the same time as the slide is trying to come back, then the possibility it doesn't cycle the slide at the speed (relative to the frame) that it needs to.
Again, I don't think it is a general statement of the 26. While the 26 is smaller than the 19, it is still generally regarded as a reliable format. But the G4, even the larger guns like the 17 and 19 have been having some issues. She could probably fire my 2 decade old Glock 26 just fine. But her newer one might need more breaking in until it becomes more forgiving of a variety of grips and stances. In the meantime, she could try a few things to see if that helps her.
Just some ideas. Also can think about giving her the 19 that does work for her right now.