Just FYI, I did searches on "overweight passenger", "overweight", "fat". I couldn't find a single thread devoted to just that topic. I found lots of threads about overweight baggage.
But maybe I didn't look hard enough. And it never occurred to me to search "POS" or "passenger of size". I'll try that.
I was actually quite surprised to find so little discussion about this topic on here. I kinda figured it would be like "booze smuggling" on Cruise Critic.
Sounds like it is!
If anyone wants to post links to any particularly good threads on this topic, I'd love to read them.
PTravel - thanks for the tips! I'll do some research on what an IDB is, and what my rights are with regards to it. I'm also going to advise my friend to go ahead and request compensation from the airline, since as you say, she had a right to the entire seat she paid for. We'll see what happens.