A few observations that may or may not be relevant to this accident.
Pressurization or no, at FL250. a functional oxygen mask should be all that is needed to stay at FL250 until ATC feels more cooperative.
But if the oxygen mask isn't working..... you might get what happened here.
A pilot not declaring an emergency followed by an unconscious pilot.
With the valve off at the tank, but pressure still in the lines, you might get what seems to be a good functional check on pre-flight.
Aviation is still very unforgiving of oversights.