A great tool to help in such cases is the router emulator, which is really just a demo version of the router's admin interface that looks and operates like the real thing without being attached to a real router. Quite a few manufacturers provide these, but they rarely make it easy to find them (TP-Link is a notable exception) on their support websites.
|The Trendnet emulator for one of its access points|
Of course, it helps if you can find the correct product-specific emulator, but in fact as most manufacturers use a generic interface for all their products, this isn't as big a problem as you might think - the menu structure is usually identical, as are the basic settings. Product-specific settings (such as a second Wifi radio) might not be there, but there's usually enough similarity to enable you to help guide someone through using the interface.
I've been looking around for emulators for a while, and I've come up with the following list of manufacturers who provide them. It's not comprehensive, and usually the latest models are not there, but it's a start. If you know of any others, feel free to leave a comment.
No special software is needed to use these - just point your browser at the correct link and the interface should appear. And don't worry, you can't harm anything, so feel free to mess with any settings you fancy. It's also a good way to check out what a particular manufacturer's router interface looks like before you buy a new router.