In a 10 gallon he wouldn't be able to make a DSB anyway. Just 0-1" for deco.
Purigen would exhaust too quickly, to be efficient. I guess if you bought a bunch of it so you could fill a couple bags to switch out while you were cleaning renewing the old stuff. Purigen work with a small bio-load. But then you would need to develop a very precise measument and changeout schedule, as you discover or add more life.
With a tank that small and no skimmer, he would have to go FOWLR and a couple inverebrates. If he doesn't do live rock then he could go FO.
You can still do a tank with some life just to get a taste of the SW part of the hobby.