Okay, so what I've learned just in a quick skim of the other threads: if the armrest doesn't come down, I have the right to demand either another seat for me, or for the POS. And if they can't accommodate that, then it's the POS who should be removed from the flight, since he/she can't be contained in one seat and should have purchased two. Or, if I have the time, I can volunteer to be IDB'd and take a later flight, and receive some form of compensation and/or class upgrade.

Also, it's crucial that I deal with the situation immediately - not after we're in flight.

I think I have a better understanding of my rights now, should this happen to me. If it does, I'll use the armrest litmus test and take it from there. And I'll pass all this on to my poor squarshed friend. Thanks!

(Mods, if you want to close the thread to avoid "shtuff", feel free - I see there are plenty of other threads filled with information.)