Leaving notice by contract

Hi,

someone I know has been working in a restaurant, where the contract gives a 3 months leaving notice, and when notified a resign, the employeer didn’t want to shorten it by any time.

However, googling about it, we found this website, which says, “minimum” being 4 weeks. He is asking if that means 3 months would be ilegal period, even though I think “minimum” just points out that contract could define more and would be priority over law (because it’s more than minimum). Does that make sense? Does anyone have experience on this matter to say how law applies in cases like this?

[https://www.arbeitsrecht-berlin.de/kuendigung/kuendigungsfrist-arbeitsrecht/](https://www.arbeitsrecht-berlin.de/kuendigung/kuendigungsfrist-arbeitsrecht/)

thanks