It’s not that version control is the only viable option, just that not using it suggests he’s not doing many other things.

As you say there’s quality of life improvements to be had. If he’s not aware of them I suspect he’s reinventing the wheel in new and interesting ways.

The devil is in the details. Being able to see a comment history alongside file level changes is very powerful over time. Version control is priceless.

Tagging changes with messages is a huge plus. Seeing when each line changed instead is another. Being able to group changes into bundles/commits is another.

Even on solo projects with linear history some sort of version control is valuable.

I would still use version control since it's literally a zero cost addition and can help with things like bisecting issues. Sometimes I do mini self code reviews via "git add -p" anyways too. Plus it's easier to copy the files/folders from one spot to another without losing any files to data corruption or forgetting to copy a file.