If I were in the left seat, I think I would look in the Airline Policy Manual and find the closest match in policy that applies to this situation. There is probably not going to be an exact remedy for this exact situation in writing, (I don't think an airline would risk possible bad publicity over possibly discriminitory policies) so I would find the section that prescribes the policy on passengers out of their seats. Hopefully there is a little guidance in there to base my decision off of. I would tend to probably try and find a way to say no, but I would want to have airline policy backing my decision. Basically I just want to cover my own ass with company policy....
So if there is litigation or bad press as a result of my decision, I still would have made the right decision.