Without having a PA31 P-chart in front of me, I suspect that the initial loss of altitude would have been involuntary, because even in a brand new a/c with expert pilotage, and the live engine going max-continuous power at blue line, there would have been descent from 7,000' down to the single engine ceiling.
We are not talking about a seminole or dutchess here with normally aspirated engines. This aircraft flies in the flight levels and has a book single engine service ceiling of over 10,000ft. If it can't maintain altitude at 1500ft close to ISA on one engine there is a serious problem with it. It would not take superior effort in a straight forward engine shut down and feather to maintain altitude or keep rate of descent under control. That descent rate assuming a few things such as blue line of around 110kias and nil wind averaged around 1000fpm, higher if he was at greater speed, it was not a drift down.
I believe something else has occured to lead to this outcome which is not clear from present evidence. Power loss on the live, fire, airframe damage, gear/flap extended, control difficulty etc, etc.... maybe the secondary issue did not manifest itself until too late or maybe the situation was underestimated, only one person knows the full situation and they are not here to tell us.
For an example of an unexpected situation which led to a two crew turbo-prop loosing similar height in very quick time read the accident report for ASA flight 529 (emb-120). The Brasilia did not make it to an airport 10 miles away from 9000 ft above it with one engine capable of full power. Had there not been survivors who witnessed the damage to the prop and wing the investigators may have struggled to find the cause, even the crew were too occupied with control to realise the severity of the damage until much height had been lost.