In my opinion the OC should not pay to paint the extension. The by-law contained a maintenance provision and therefore the OC is not responsible for the painting of the extension.
I am assuming that the owner paid for the initial painting of the exterior of his extension without requesting that the OC pay for it. If the extension is only relatively new then it will probably not require a repaint at this time, therefore the owner should not have to pay any additional painting costs at this time.
Perhaps this can be the “trade off” in this instance?
When it comes time to paint the building both now and into the future your OC should just paint all of the other walls of the building and leave the extension untouched.
If the owner doesn’t believe that this is a ‘reasonable’ action then they have the option to lodge a dispute