First I've heard of this but for years I've watched 100s if not 1000s of ED/Ambulance/Air Ambulance TV programmes where they give ketamine at least twice an episode so that the Clinicians can pull bones into place while the patient is off their head and the weird thing is the effect doesn't last long so once administered they have to do it quickly before they come around.
Matthew Perry must have been proper hardcore.
It doesn't need to last long for someone to slip under the water in a hot tub and drown. He might well have been proper hardcore as you say or he might well have been a regular user of a more moderate dose which should never have been prescribed so easily and should have been more carefully monitored, restricted etc..