Privacy

Probably a bit better than Windows but ultimately with Apple you can't know for sure. Their software is proprietary and closed source, so it cant be properly audited by a third party.

Apple can claim whatever they like about privacy, it comes down to whether you take their word for it or not. Personally, i think Apple has been caught being dishonest about their software more than most tech companies, so i take their claims with a heap of salt.

If you want something that can actually be proven to be secure, the only real option is an Open-source (and ideally FOSS) platform. Anything that can't be audited can't really be trusted.

For a laptop or desktop, Ubuntu is a safe, secure, well-audited option. Mint and Fedora are also good options for most hardware. There are good DAWs available on Linux but perhaps not the exact one your friend is used to.

I would suggest anyone new to Linux to back up or remove their ssd, and install Linux on a second or new ssd to reduce the chance of accidentally wiping your data. A new OS is like getting a new computer, and you need to save your data externally before you start the process.

Edit: I'm aware that some parts of macOS are technically open-source, but that's almost meaningless from a security perspective when critical parts of the OS aren't open. Darwin being secure doesn't make the whole OS secure when network manager is still a black box (for example)