It is abusive and largely illegal but that's irrelevant. Once you sign something like this, even a small company has enough resources to harass you with lawsuits that you might win after a few years but by then your life is mutilated.

What they gain with such threats is control over you with an installment of fear to ask for a raise or do anything that separates you from the company because that clause makes you unemployable for X years while it's active. The former employer (O) just needs to check your LinkedIn profile and send a notice to the new employer (N) that everything you write for N must be submitted to O so it can unilaterally decide if it actually belongs to them. It's absurd but I can believe companies try this because they have nothing to lose.

He refused the offer but I would like to know how common such a clause is, this is why I ask. Until now, I haven't thought about how careful you have to be when signing what one could assume is an absolutely benign and common work contract.

Depends on EU country of course, but any court can invalidate such clause or even entire signed employment contract without much effort, especially in such highly unilateral uncompensated unbounded case.

I am sure refusing the contract is tough decision for some but I think in a long term it is a wise move. Companies with such shaddy tactics usually are quite bad toxic places to work in and lack any reputation. Just check some other non-compete/confidentiality clause threads here in HN. You will find similar experiences.

From my experience it is extremely rare situation in EU nowadays, but I may be biased as I work with reputable companies from my network recomendations.