The Saiga-12 has 15 parts that count towards 922r compliance. The magic number is a maximum 10 foreign parts.
US Parts (7):
Buttstock
Pistol grip
Fore-end
Gas Puck (I'm guessing, from the parts tubes)
Magazine Body
Magazine floorplate
Magazine Follower
Foreign parts (8):
Receiver
Barrel
Front Trunion
Bolt
Bolt carrier
Trigger
Hammer
Disconnector
So as long as the mags in use are US made, he's good to go. If, however, he uses the Russian made mag, 3 parts move from the US column to the Foreign column, and the Saiga-12 becomes non-922r compliant. Lovely, ain't it? :banghead:
I think, if he were to simply swap out the hammer, then the mags would not be a problem. Then again, I don't know if any of the compliant hammers would work with the factory trigger and disconnector. The best real reason to do the conversion, is that the factory trigger leaves much to be desired. Once converted, the trigger is surprisingly nice. I prefer it over the stock combat AR trigger.
Edit: My guess is that the 10 rd mags are ProMag. It's a good enough range mag, but there is enough anecdotal evidence showing that they probably aren't Duty worthy.