Linux won't Quicken for you (a tragedy, I know), but interop with scanners and Microsoft office is easy. Libreoffice has had really straightforward docx support for years and the printer/scanner driver is literally the same one MacOS uses (CUPS).

It's still a nuanced decision and I agree with your original conclusion. That being said, if you're going to assist them with setting up any machine then I don't see why Linux would be particularly difficult (besides the Quicken-ing).