Agree - CO has a policy which should be enforced. You should never be forced to sit next to someone who must have the arm rest up.
Either they buy an extra seat or out they go - that simple.
I was on an RJ from CLE-DCA last month - and a very large woman boarded. She had purchased two seats. She needed two seat belt extensions. But she had done exactly what was required - so no issues at all.
Even though there were empty seats - she planned ahead. I was next up to have my boarding pass scanned when she arrived in a large wheel chair with a friend . The friend handed over 3 boarding passes for the 2 of them.
On the RJ they were across from each other - the friend in the A seat. I was 12A - they were 13 A/B-C. She made it made it back - sideways - a little tight in the aisle but still made it.