Sounds like a good way to kill yourself and a student pilot who doesn't really know any better.
While it isn't 'law', CAAP 5.23-1 about multi engine training states (para 4.17.9) :
"Propellers should never be feathered in flight during training below 3000ft
AGL"
You'd be a brave person to go against such recommendations. Previous flying schools and CAR217 organisations I have worked at stipulated something similar in their operations manual.
I'd say your 'fairly experienced' mate is either an idiot or has miscommunicated what "fully shut down" means. Closing the mixture doesn't mean fully shut down, but feathering the prop as well pretty much does.
Good luck getting it unfeathered when the (very likely) day comes that you need both engines. I've had many a unfeathering accumulator work poorly or not at all following a feathering exercise >3000ft.
Setting zero thrust is a MUCH more sensible option (and it doesn't matter if you cut the mixture or close the throttle as your failure technique). If you really want to demonstrate the 'reverse yaw' on touch down, don't close both throttles during the landing - leave that tiny bit of power on the 'failed' engine.