Not that he should try playing BFBC2 on his computer, but you set the render ahead limit to 0 in fps games to eliminate mouse latency (along with disabling vsync, and any d3doverride-triple buffering if the game supports it). This goes for all fps games. One thing to note is that when you buy a game, you need to make sure that your video card in its default configuration is able to run the game at your desired level and speed. If you want to turn render ahead to 0, that increases the minimum threshold for graphics card requirements sometimes by as much as a full tier (depending on the quality of your hardware). Conversely, if you are experiencing single-card micro stutters or hitches, occasionally turning Render.Ahead limit to 5 (up from default 3) will alleviate the issue mostly/entirely.
The dx9 thing is a BFBC2 specific fix that can help the game run faster and some people prefer the look of the game b/c it helps get rid of some of the ridiculous HDR and Bloom, as well as making character models stand out a little more. This is mostly relevant to people with bad computers, or people who are extremely serious about competitive BFBC2.