In this case even if a CC person were on the flight deck what are the chances of them realizing what was happening in time to prevent this. I understand that she/he could have possibly let the captain back on to the flight deck but based on what has been reported it would seem that the alarm was raised only after the captain could not regain entry.
As has already been pointed out its all pretty moot anyway considering that if either of the two people at the front decide to end it there is not a lot that the other pilot, CC or SLF can do. Like anybody in control of people carrying transport the public places trust in the individual/s at the pointy end.
It's the trust in the system (and fellow workers) that allowed this individual to be in his position that I suggest should be questioned.
And no I don't have the answers for that.