Also I think 1280x768 is not an ideal aspect ratio (the image would be slightly skewed) - it might effect where the game considers the screen edges. Try 1280x720 instead.
If he is using the TWM_GUI mod with vanilla BG1, then 1280x768 is the only option for a 16:9 monitor.
But I have used that combination & it scrolled correctly.
If this is what you are using Gibush.
You need to install the widescreen mod first @ 1280x768 & don't choose support for multiple resolutions.
Also check you haven't got your Scroll Lock key on your keyboard on.