I am French and yes it might be illegal under circumstances. For example if someone is paid for a service (however small the amount), and taxes are not paid then it is illegal [0].

It might looks quite easy to understand but it is not.

For example a local newspaper owner was handing out flyers at the entrance to a stadium. He asked two kids from his family to help him and give them some money. Two tax inspectors who happened to attend the sport event, issued a ticket of several thousand euros for "illegal work". Yet the newspaper owner had no such revenues. The "newspaper" was a just a hobby.

(I tried to find a link in the press but failed to find it)

[0] https://fr.wikipedia.org/wiki/Travail_ill%C3%A9gal

That's different from the question, IMHO.

It's not illegal to "voluntarily help someone who is not family or to barter services" or even (obviously) to be paid for work. But any of that may give rise to various tax liabilities and not fulfilling them is illegal. I would think that this is pretty much the same everywhere in the world.

I don't think helping your neighbor move some furniture without compensation is going to create a tax liability.

It sounds like there was some payment involved in that case.

The OP's statement included "it is illegal to voluntarily help someone who is not family", which would make it illegal to help a friend move furniture or paint their house (without remuneration).

You are right indeed.