That's not entirely correct, to be honest. Every Linux LTS version has some kind of end date. Sometimes modern packages start getting unavailable even long before that and you need to install custom sources and whatever.
Of course, you can upgrade to a newer version, but even that is not 'forever'. See: https://linuxiac.com/linux-kernel-developers-discuss-dropping-old-cpu-platforms/

If he does end up choosing a Mac Mini, that's an expensive, but not terrible, relatively low-power, but more capable platform than e.g. a Raspberry Pi, so he would have more computing headroom for other stuff.

One more thing regarding longevity: To the best of my knowledge, Apple hardware gets major version updates for many many years. Even when that stops, similarly to Android, they still ship security updates. For instance, macOS 11 is still supported, with its first release in June 2020.

And lastly, even when that support ends, technically Bitcoin Core will still run on it. Bitcoin Core doesn't really care whether your OS is still updated or not. Although there may be security concerns at that point. Even then, the Mac Mini buyer can still always install some nice, modern LTS Linux distro on that hardware...