As far as I am aware, the Commander is ultimately responsible for the aircraft and its inhabitants safety. To achieve this, he is allowed to make decisions and orders as he sees fit, in case of an emergency even above and beyond the scope of the books and the law. In emergency, he is only measured by the result: did everybody make it out alive or not? If the company entrusts a Commander with an aircraft, it at the same time entrusts him with this power.
There is no instance above the commander on the aircraft. Should there be any doubt, a look at the Polish Airforce Tu-154 accident in Smolensk might be enlightening.
I would be highly surprised if this termination was to be held up in court and not kicked out unceremoniously. All the best to the Captain.