It could be that it works similarly to Croatia - it's not illegal, but if you are found to be "a hidden employee" you simply owe taxes and retirement/healthcare contributions as if entire income of your single-employee company was your salary.
It could be that it's actually illegal in France. But many people are confused about how it works here in Croatia, so it wouldn't surprise me if it were the case in France too.
> The "employee" creates a single-person company in Poland
Is it really illegal in France or do you just owe more taxes/contributions than if you had multiple clients (which would allow you to take part of the revenue as company profits, instead of having to channel it all as a salary)?
Looking at your comment [0] it sounds like it might not be illegal (provided you pay the required taxes/contributions)?
1) The company opens a branch in France, with as director and sole employee the person in question. Employment contributions are collected on the money that the French branch pays to its (sole) employee. Very simple to set up, does require a bit of work from an accountant for the yearly accounts (I'd say to budget about 1kE/year)
2) The employee joins an IT contracting company that already has a commercial presence in both countries, they will take 10% of the take (known as "portage salarial")
Those contracts are of course possible and exist - although rare due to paperwork hell you need to go through. The labor law of the employee's country is observed and they usually need to handle pension/health contributions on their side. In most of EU at least.
It is true though that most EU countries also demand that you open a local subsidiary once you have a certain amount of employees there (~5-10).