My Australian roommate needs to stamp his passport to prove he has been out of schengen.
There are many opinions about whether or not this is still accepted as a renewal of your visa in Poland. This is not the nature of my question.
What I need to know is what is the fastest, cheapest, easiest way to leave schengen and return?
The passport stamp won't prove anything - the Schengen rules are 90 days in 180. He might get lucky and they won't check his passport - but given that they always look through mine when I request a stamp (as an EU national!) - they're not going to miss someone overstaying on an Australian passport.
There's no such thing as 'opinions about whether it's accepted' - Schengen rules are fixed, and if he's caught anywhere in the zone having broken the 90/180 limit, then he'll be in serious trouble. It's even more serious now given that the Straż Graniczna are actively stopping people anywhere in the country for random 'border' checks - and the maximum punishment available is a 5 year ban from the Schengen zone.
The 'safest' way would be to go to the UK. If he's refused re-entry into Schengen, then there's always the option of going via Dover - which has notoriously lax entry checks from the French. But again - be warned, a refusal from Schengen is marked in the passport - and this can often be grounds for a future refusal.