Unfortunately, it's not so simple. If he's working (even if unpaid) for her business,
I didn't say he has to work for his wife's company- why would he have to?
If, and that is only "if" he is registered as an employee, then they will have to pay full ZUS for him.
The minimum earnings are declared by the owner of the campany- his wife, who is also employed somewhere.
If she declares minimum earnings for a company she is running while employed somewhere, they are not too bothered about her husband who,in this case, will be unemployed in Poland.
This is not theoratical. It's based on the experience of my colleagues I live with in my area.
He's asking for a control to take place and for him to be hammered.
He works in Germany and he pays his taxes there.
How he spends his money between Germany and Poland, is up to him.
10 years on- no one has has ever knocked on his door to ask if he is paying Polish tax on the money he earns in Germany.
To the OP: if you or your wife have friends or relatives who are self employed in the UK and also spend time in Poland, ask them how it's done.