I really apologize, Jules. I have been trying to play catch-up from being gone on family business for a while, plus I wanted to think a little on this.
I think that the way I would do it is to start with a light wash down with dishwashing detergent (Ivory is a good choice) and water. Use a damp sponge for applying and paper towels for wiping it off. One problem with shellac is that things like petroleum based solvents, alcohol, and alkali's (like TSP) can harm the finish. I would expect that there is some wax on the surface but the same things that remove wax also remove shellac.
You don't want to let the surface of your cabinets get and stay wet but a quick wipe with a damp sponge should not do much harm. If there are cracks and voids in the shellac, water can get under the finish and lift it, so start with a small area and watch that for a day or so before you proceed.
Many years ago, I used the kind of process that your friend is describing on some old finish but I don't recall if it was shellac or not. Homer Formby sold a kit for this and I don't know if it is still available. In any case, I thought that the process was a little rough on the finish and did have a tendency to mix the surface dirt and wax with the old finish. I know that what your friend has described is possible but I don't really have enough experience to give you any "ground rules". I was not that pleased with the piece that I tried and ended up stripping it anyway.
Rather than this, if the shellac is still in pretty good shape, you might try a coat or two of Johnson's Paste Wax a couple of days after the wash. This is probably the best and safest choice, in my opinion and without seeing the present finish. Follow the directions on the can. Be sure that you have plenty of soft rags for the buffing.
This may not be what you want to hear. The Furniture Guys, Ed and Joe, did a segment on this but I have not caught them on TV in a long time. You might do a search for them on your favorite search engine or just try www.thefurnitureguys.com or www.furnitureguys.com and see what comments Joe Lerario might have. Wish I could have been more help with an easy answer.