The flight attendant can put it in an overhead or closet (if there is space). There shouldn't be a problem. If she needs it during the flight to move around the cabin, you can get it back out.
Besides what the others wrote, I wanted to mention that a mobility device doesn't count as a carryon - she still is entitled to still have her regular carryons.
Our Authorized Disney Vacation Planners are here to provide personalized, expert advice, answer every question, and uncover the best discounts.
Let Dreams Unlimited Travel take care of all the details, so you can sit back, relax, and enjoy a stress-free vacation.