Not sure where you originate from but suspect it's Stateside. In the UK ATC Managers, Supervisors, Bosses, etc., do not interfere with the operational controller unless there is a seriously good reason to do so. Quite often those people are not qualified to do so, ie they do not necessarily hold validations for the units where they work.
In answer to whether one should intervene - been there, done that, got the tee shirt... and seen it happen several times with other controllers too. My attitude - and, I suggest, the attitude of all professional controllers - is definitely NOT one of watching the bloke make a cock of it. Whether there is anything definitive in writing which says we should take action in such circumstances the answer is probably "no". But if safety of human life is involved then it's our job to take action and if a crash had resulted one can only imagine what the lawyers might have said! If the guy wants to whinge later, that's when the supervisor does his bit.
It's not only controllers. I have first hand experience of a runway checker vehicle, not driven by ATC personnel, being on the ball and warning ATC of imminent danger to an a/c. ATC subsequently took action based on that info and saved a lot of lives.