If he's putting that much air in the shocks, I seriously doubt he is bottoming out. A more likely case would be that the shocks are SO POOR on these bikes, and the front end suspension as well, that it FEELS like he is bottoming out when going over those sharp edged bumps in the road that are everywhere I've ever ridden. The compression damping in the stock suspension is so bad that the suspension cannot react to those sudden changes/input. That goes for both the front and rear. All the air does in the rear is provide additional "preload" to the shock where it won't bottom out, but in doing that, it makes the shock feel really "stiff", like it has a super duper heavy spring. The cheap way to do suspension on a bike is to have simple holes in diaphrams within the shock or front suspension, in addition to a POS spring in each fork leg...the suspension oil must be squeezed through these orfrices, and neither the oil or the size/action of the entire set up can react quickly enough to absorb those sharp edged bumps (like the ones where the road meets a bridge)...in other words, only a certain amount of oil can be forced through the holes...it works OK on dips in the road surface, but can never react well to sudden changes. In fact, once you put on decent shocks and rebuild the forks with SOMETHING better, even if it's just a better emulator, you'll realize just how much the front tire is skipping around on the road surface...not maintaining constant contact. This can make for a giant pucker factor if you encounter rub board type pavement in the middle of a curve. Traxxion Dyamics AK 20 cartridges are the very best solution for the front end, but it is relatively expensive...the Progressive Monotubes/new springs are probably the next best, though I have no personal experience with them yet...and new/different emulators coming in a distant 3rd. I have no vested interest in Traxxion Dynamics products, other than knowing they are the best, and the components are machined in house to extremely high tolerances (the AK 20 cartridges). Traxxion also offers a compromise...they can rebuild the forks with better springs, and a far better emulator, with the proper fork oil. They are also excellent folks to deal with, and really know their chit, coming from a racing background. If you live anywhere within riding distance of Atlanta, it's the best money you'll ever spend on your 30K plus motorcycle.
On the other hand, folks don't think a thing about spending 2K+ on the HP/TQ quest, like cams, trying several exhaust systems, tuning devices, etc, etc, etc to make the bike go faster in a straight line, but will balk at spending 2K on suspension to get the power you already have down on the pavement at all times, and be SAFER in the process. Most HD owners don't really know they have chitty suspension, as they've never had anything really good to compare to.

Same used to be true with HD brakes, but they finally got around to putting something decent on the bike to stop the damn thing.