This thread took a harsh and icky turn. Some people legitimately can't help the fact that they're fat, for whatever reason. When that's the case, there's usually an underlying medical problem going on and I can respect that (and sympathize with it to some degree--goodness knows, I've got my own set of health problems to deal with).
But it's just that: a problem. Not something to be rationalized away as "okay." It's not healthy, and I'm tired of seeing people who probably just need to get off their bums or eat less try and push for "acceptance" of a problem that should make you uncomfortable. Is there an issue with society having an unrealistic and often unhealthy idea of what a healthy size is? Yes. Not everyone is healthy when they're stick-thin, and too often, that's not the size they're meant to be and pushing it causes more harm than good. Does this mean that obesity should be acceptable? Absolutely not. Obviously I'm not for taunting people because they're fluffy, but being big and out of shape is something that should make you uncomfortable. Now, this is a rant that's only tangentially related to this story, but I'm tired of hearing people try and force the notion that obesity is something that needs to be accepted. As with all health problems, it's something that needs to be handled with a bit of tact, but it isn't really a condition that needs to be excused or celebrated, IMHO.
I have no idea if the passenger in question has some sort of issue that's preventing her from being a more healthy size. I can see the need to handle that situation fairly delicately just in case there's more than just laziness/eating too much crap at work there. It doesn't appear to have been handled very tactfully at all, though--hence the problem.
But for pete's sake, I agree with what another poster said--if you require two seats, you probably should have to pay for both.