And not only that, but a verbal contract cannot modify a written contract. At least in US law. So he is only bound by what is written in the contract he signed, not what was said outside the bounds of that contract.
In, say, Germany a verbal contract is as good as a written contract, as far as I know. (And both situations aren't really applicable to the Netherlands, anyway.)
I wasn't saying a verbal contract is not as good as a written one, I was just saying a verbal contract cannot amend a written one. ie. If he signs a contract stating he can work independently on nights/weekends, they cannot amend that contract verbally and change their minds. Amendments to written contracts must be agreed upon by both parties in writing. Though I'm not sure if this applies in the Netherlands...
In the German system the oral addition stands above the written contract. The written contract is considered boiler-plate by the courts and the oral addition is more specific, and bears more weight.
Of course all parties have to agree to amend a contract. So just the employer changing their mind wouldn't cut it.
The Netherlands is a civil law country, so the German standard might be closer to what they have than the common law American standard.
Labour relations and law are of course a strange thing in themselves.
I understand non-compete clauses, but this doesn't seem to apply here.