Lumbar canal stenosis .....I'm not nitpicking but...
You question is if you could prolonge the sitting time. Why can't she sit longer than 10 minutes? Does she sit/stand in a certain position/posture? How about her pelvis levelled? What about a more resent Wikipedia reference-linkMRI?
I assume because of pain/discomfort. In that case you should palpate the gluteal area and lower back for Triggerpoints in this; All gluts (if they have active triggerpoints sitting & lying on the back may provoke pain), paravertebral high tension of muscles could irritate the nervous system. Quadratus lumborum & Latisimus Dorsi. Irritation of the nervous system could result in irritation of other organs like muscles whci then could effect movement patterns.
An active point you will find without problems follow the charts and ask about referred pain (but don't get fooled if it doesn't correspond with the books) The pain on touch should be extremely sharp, not dull (otherwise you might irritate more organs e.g. muscles) and should start to ease within 10-20 seconds. Do not continue if they don't you might be just aside of 1. Palpate again if necessary. Don.t push to hard, unnecessary to do so. Gentle does the trick.
Best of luck & your friend as well.