It's all I care about, unfortunately.
I want Proton to succeed simply on an ideological basis, but myself and a lot of other people are being severely hampered by this lack of functionality on the Linux desktop, which is ironic given that this is where the privacy-centric user base lives. As a customer of both Proton Unlimited and Proton Business, I don't feel taken care of, and almost all the alternatives have some functional, easy to use drive sync functionality on Linux, on top of letting me use the cloud calendar locally.
I'm slowly migrating away from Proton Business use towards hosted mail + NextCloud as it did not meet my needs at all for a 90% Linux desktop use case, but I hope to revisit it in the future.
Definitely, he should try using Linux in a typical Microsoft-driven corporate environment with a regular laptop for a month and see how it is.