There is a very small chance that a DS virus is named so that it is thought to be a game, most PC virus checkers will pick up on it as soon as it is downloaded. There isn't really any DS virus in circulation, there has been one created but it has never become really mainstream. There is a program which will check any NDS files before you put them on your DS (can't remember what it's called as i never used it). You can protect your friends DS from viruses by installing FlashMe as this writes a recovery system to a protected area in the flash which a virus cannot access, however installing FlashMe does have it's own risk of bricking if you don't follow instructions properly. In the whole time I have ben in the DS scene (pretty much as soon as it was hacked) I have never downloaded a virus for it.
