There are lots of problems with the adapters. I would not waste the time or $ trying to use one. Besides, if he ever wants to sell that sweet '03, the value will be markedly diminished unless the adapter is removed (which has its own set of problems). Even if going the 30-06 to .308 adapter route was successful, the difference in recoil is negligible, so you won't have gained anything.
By far the best thing to do is start reloading, as noted above. With proper selection of powders and bullets, it is child's play to create low-recoil, safe loads for .30-06. Plinking loads can be be made with virtually no recoil (especially if you use light cast bullets), and hunting loads with only minimal recoil. A single-stage Lee setup is all that's needed for the relatively small volume of shooting he'll do, and the whole thing can be had for about $200 ready to roll.
From the standpoint of preventing a bruise (which is, after all, the whole point of this): contusions occur because of localized stretching of small blood vessels, causing them to tear. If the force is spread out over a larger area, there is less stretch on the vasculature, and therefore, less tearing and less bruising. One of the PAST recoil shields would help a lot in this area.
Adding weight to the rifle will also help dramatically, whether it's integral to the rifle or as part of a rest like the aforementioned Lead Sled.
Muzzle brakes or porting will not be terribly useful. They work best when there is high gas pressure, and light loads will not develop sufficient pressure for the brake to make much difference.