Is it safe to still use an iBook G4 on Wi-Fi?
What your friend is likely talking about is not connecting it to the Internet as the newest macOS that works on that Mac is OLD.
And that matters because it does not have the benefit of updated patches to macOS that make it more secure and more importantly it runs a badly outdated version of Safari that may not even connect to most modern websites due to standards changes in web publishing and the outdated security certificates built-into old versions of macOS.
Depending on which iBook you have you may be able to install a more modern browser. I have an old G4 tower with some older, never updated, apps on it that I use from time to time and use a version of Firefox called TenFourFox. It (as I have been told) supports G3 and G4 processors. So that particular browser would be safe to use due to its more modern foundations, if slow on modern websites.
I believe there are other, alternative, browsers for legacy versions of macOS, but I have no direct experience with them. Other folks here may be able to chime in and offer suggestions, but it would be good if you edited your original question and add the model of iBook you have and the version of macOS installed on it.
But without a more modern browser I would also suggest you avoid connecting to the Internet, merely for safety reasons. Wi-Fi, itself, is likely okay as long as you are only using it to transfer files between your local Macs and not going out onto the Internet.