My understanding is that the PF was in the left hand seat and the PM was in the right hand seat? Realistically it wouldn't matter either way which seat he took.
What is important is that the PF was unable to manage the situation in the aircraft commanders absence. Upon his return the aircraft was at risk and in an unknown and unstable condition.
Common sense dictates that in crisis the most experienced flight team needs to control the aircraft. further the PF has demonstrated by the planes condition his inability to manage the crisis. So the correct and logical thing to do is replace the least senior (and demonstrably least effective member of the flight crew).
Had he immediately instructed the PM to assume command (regardless of actual seating) and replaced the PF he would have had the best combination on the flight controls and given himself (and all souls onboard) the best chance for survival.
I'm sorry but any command pilot who would act instinctively in that manner should be driving a bus...(and I mean a real one).