My ds is a big lad - yes he could do with losing weight (work in progress) but he's still always going to be over 2m tall & built like the proverbial brick shit house. He manages to contort himself into extra legroom aisle seats without encroaching on others (I've sat next to him, it's fine & I am not tiny), but airline staff do tend to seat him next to an empty seat where they can.
I've often thought there could be a market for 'comfort' seats - 2 in the row, significantly wider, marketed at bigger flyers, people with squirmy toddlers & miscellaneous spreader outers, for 150% of the regular price.
Not as expensive as paying for 2 seats, more comfy I'd imagine than having your bum spread across 2...
Yes I do know Business/First Class already exist & are roomier seats than Coach. But then you're paying for all the other bells & whistles.
Anyway, she definitely WNBU. She could have paid for the seat for her size, her favourite handbag or her invisible pirate ghost husband. Doesn't matter. She paid, it's her damn seat.