In the OPs case if the PAX required a seatbelt extender I would have gone up and discreetly said something to the lead. For safety reasons a seat belt extender isn't allowed in the exit row (noty sure if it's an FAA policy or just certain airlines, but I have heard both Delta and WN FA's tell passengers it is a safety issue because someone could trip over it).

If a seatbelt extender wasn't required, I would have put down the arm rest. I ALWAYS put down the arm rest as soon as I am seated anyway. If the arm rest wouldn't go down because of the POS, then I would have discreetly told the FA you consider it a safety issue. Even if you aren't in an exit row, there is NO excuse for you not to be able to lower your arm rest.