I got this shell for my DS Lite from realhotstuff, which is a clear plastic case, with silicone inserts. Nothing can scratch the DS, and the silicone seems like it would give quite a bit of cushioning even in a drop.
But anyway, whenever you lend somebody anything, always assume you're not getting it back, simple as that. If you're that anal about your stuff, just don't lend it out, because you're not going to like what you get back. There's a reason people say "Nothing parties like a rental!" Seems like you learned that lesson pretty cheaply.