to don the 02 mask when the other pilot leaves when above FL250
Precisely my thought.
The requirement for quick donning equipment is above FL250 with this being the line in the sand for TUC considerations.
At some point, operator management needs to put a flight standards line in the sand to avoid having folk setting themselves up for an irrecoverable situation.
The industry has seen several such fatal events (I can recall one civil and one military fatal in Australia in the not too distant past - and another near miss situation where a passengering pilot picked up the signs early enough to enable the situation to be rescued).
That should be sufficient to prove the need for a degree of operational caution, I would have thought ?