Thank you Liz and iflyua2 for the well thought out responses. I was under the impression that if you had a cane you couldn't sit there - but I stand corrected!

Also, in regards to the language thing: How recently did that rule change?

Last year I was on an SEA-ORD flight and when the FAs started the drink service in Y, one of the FAs realized a guy in the exit row only spoke Polish - no English. They made an announcement to ask if anyone on the flight spoke Polish - they were trying to explain to him that he couldn't sit in an emergency row if he didn't speak English and they were trying to get him to move - he was obviously not happy. No idea if they asked if he was "willing and able" before the flight or not.
So, in this case, would you have tried to make the guy move, or just let it go, per the new UA policy?