I'm not little by a long shot but I only take one seat and the only time I have flown and raised the arm rest was when my husband and I were flying next to each other. Never because I didn't fit in the seat.
If you don't fit between the two arm rests you need to buy another seat.
Maybe first class might suit her better. One first class seat is probably a little less expensive as two coach and the first class seats are bigger. I really don't know. I have never bought a first class seat.