MacOS – How to permanently disable the scrollbar

You should be able to grab the invisible scrollbars. Just click and drag near the edge of the window where the scrollbars should be and they should appear. I can't say if this works correctly for all types of windows though.

That's your reason right there, you are using a traditional mouse, and so you get the traditional scrollbar, it should always be on view. Your friend is using a touch input device, and like with trackpad users this does not rely on accurate positional use of the cursor, and so the scrollbars disappear unless active scrolling is happening, as they do not need to be seen for a target, and when they do show they overlay so as not to cause a nasty screen artifact (namely an 8 pixel wide scrollbar flashing in and out of existence, possibly causing screen repagination etc)