S.S.
if you are willing to pay?? there is business and first class with wider seats.
While I agree that it's "discrimination" to make her buy two seats? You could call the airline directly and ask them how this will work. Tell them how big she is and what she needs.
https://www.tripsavvy.com/flight-ticketing-for-larger-pas...#
Good luck. I would either upgrade to first class and pay through the nose for it. Or find another way to travel, such as train or car. If you have to buy two seats? Why not just upgrade?