Have to agree, sueing would be a good thing, but not per definition because the airline(s) are wrong!
I've had the porblems of having to transport people of odour, "vocalness" and of size... I had little to nothing to go on but the seatbelt, aisle, or cabin crew not being able to handle the arising situation! A seat size is a seat size, and even armrests are having to be shared HALF the size they were by two persons now in coach, everything becoming ever more cramped!
A private transport aranged by her insurance company (like they do on ski hollidays when a leg is broken etc) would have been the way to go, not taking the cheapest way out, and then having this result, hoping for a remuneration that will bring "justice". Bad planning on their part, and IF the airlines and/or insurance company's' didn't come with this idea too, bad service on their part too!