First question out of the blocks---have you shell sized the person? plus, boots are width sized on a 26 mondo shell and vary somewhat up and down from there.
I just measured the exterior width of a 23.5 Salomon X3 lab liner at 87mm---the interior width would have to be less. The narrow boots exist in the upper end (race level)of most boot lines. You should be able to find a boot that will work for her.
Well, I know that narrow shell = stiff, performance oriented boot, but the girl is level 6 or even less skier and stiff boot isn't the solution. We are looking for something with flex index 70/80 or so, Xlabs are much more rigid. I was thinking about junior race boots, am I right ?
PS I sized her shells ( Nordica Beast ) 23.5, there is 15 mm space but boots feel sloppy, too wide.