The cost of 2 seats may not be as much as travelling premium economy which would give a little extra room and you could also also if it would be possible to have a bulkhead seat to allow more room in front and the tray comes to the side rather than in front.
The exit seats obviously wouldnt be allowed as you have to be physically fit and (in case of emergency you may be asked to open the cabin door and also that any disability would hamper other peoples exit from teh plane quickly), but there are bulkead seats that arent by an exit.
I know that she isnt specifically disabled because she is larger but may be slower than could be necessary in an emergency situation.
She wont be the first to ask and Im sure they will have some way round it.