My friend's an engineer, so maybe not a flashaholic, but I'm not too worried about the battery aspect.
Unless your friend is a flashaholic this is a bad idea. I'm assuming your friend doesn't know the in's and out's associated with LiOn batteries.
A flashlight that runs on AA batteries is a better idea.
The S10/R is not a keychain light, thats not to say you can't put it on a keychain. You can also put the S20 on a keychain..whats bearable to you?