I can only imagine how their UX is declining day over day.
They are probably just fine. Most people are perfectly happy with Windows, we are a minority.
That said, don't dual boot for noobs. It's a pointless exercise in additional complexity. If you default it to Windows, they'll never leave Windows. If you default it to Linux, they'll be forever frustrated that it booted to the wrong OS.
Install Linux on a new disk, insert it on their box, and if they hate it and ask for Windows back, give it back. Forcing them into Linux land will just make them resent it.