You can get rust paint, which stops the rust from progressing any further. Basically, you remove what rust you can with a wire brush or file, paint it on, and then putty and paint over it.

Or, an angle grinder, and grind or cut off ALL the rust, then fabricate some repair panels, weld in, file, putty if necessary (or lead fill if you're old-school), sand, paint.

What sort of car is it? Some vehicles have rust repair panels already available (like sills, floor pans, quarter panels, etc), for a lot of classics (or some panels like sills or floor pans can be cut down and modified to fit a different car). Or, take him down to pick-a-part, or jollys, or somewhere like that, with a few tools, and he can remove the good parts he needs from another car. This is a great way to get interiors, and little clips that you always seem to lose at home, not just panels. One note for those places: NEVER LEAVE YOUR TOOLS, EVEN ON THE OTHER SIDE OF THE CAR YOU ARE WORKING ON. KEEP THEM WITH YOU! But, having said that, they are great places to take cars apart.

PM me if you want, and I'm sure that DS16 can correspond via email with the Man on the topic. The Man has restored numerous cars, and I'm sure he wouldn't mind passing on some of his knowledge to another eager student.