His computer doesn't seem to be the problem here. I had the same problem, you need to verify your game files. In Steam, go to the library (the list of your games), tight click on Rust, go to properties (the last one in the list), in local files (third tab) chose verify integrity of game cache (last button). It takes a few minutes during which Steam may freeze, after that restart Steam and you're good to go.
Rust is also not optimized compared to finished, released, "good" games. You're comparing an alpha to something that's been through a QA cycle and a public release.