Sarvan,
I asked my wife about this. She worked for several years in dialysis centers.
The shin guard would work but not attached to the sleeve... too great of a chance of slipping out of place and causing a problem. If you can find one that is close to the size of her arm and place some dense foam on the top and bottom edge (the curved part) where it will hold the guard away from her arm and not rub, then with elastic or Velcro straps or an Ace bandage attach it to her arm so it will not slip would be best. Also she said be very careful about the heat and watch her liquid intake as they are limited on how much they can consume. Oh and she said tell her to have fun and be safe. Also please let the ref's and any medical staff at the event know her situation, it helps us be better prepared in the event there is trouble.
