OS X Lion Safari and Chrome's Vertical Scrollbar (overlay versus next to content)
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)