Heyho, recently someone asked for the silliest reasons, but as someone who has suggested linux to many people, I often encounter people having valid reasons for staying with Windows or switching back.
The most boring but valid one is “I have to use Windows for work. It is a requirement (of some software I have to use)”. But there are also other answers that fit. My sister for example tried Linux, but while installing software constantly encountered issues that I helped her solve and eventually switched back because she felt like she had less control than over windows. While I am aware that this is fundamentally wrong, it is valid that some amateur users do not want to invest enough time to get over the initial hurdles of relearning how to install software.
What are the best reasons people have given you for not wanting to try Linux?
Depends what he’s looking for. I think Onshape (browser based CAD) has a free version. Your data is public though unless you go with a paid version.
If he wants a free Linux CAD there is FreeCAD and a few others.
If he is attending a university, as some retirees do to audit courses or enrich life, then Siemens NX (what GM, Stellantis, SpaceX, etc use) have an academic license for around $100 a year. It is now Windows only based, unless you run Linux headless version, but if you use any version NX12 or below there is a GUI LInux version that runs on REL or SUSE (or openSUSE since it shares SUSE binaries)