That may depend on how you’re using it. You certainly can add a windows share as a remote and push to it just like any other remote. You’re not going to get anything fancy like hooks/ci/cd of course but it works. Though I can’t imagine the performance is great.
If you’re each using that directory as your own checkout, that’s pretty bad and likely to result in problems. I would avoid that.
There are definitely features of a real git server like protecting branches from force push, accidentally deleting, tracking issues, etc etc that you’d gain from even a basic GitHub repo. I’d still push for that.