RL 16 is another very versatile powder as well. I would consider that if you are having trouble finding 4350. Even Varget could work well depending on the application. I would say whatever you are thinking, get at least 3 pounds of it so that there's a chance to shoot and learn, with experience being the biggest teacher.
I would also suggest that if he has a 223, that it is probably the easiest cartridge to learn to reload on. With an appropriate powder, it's almost impossible to overload one, it takes far less powder, the brass is available EVERYWHERE (just go to the range and pick it up! It's free!), bullets are cheap and plentiful and you'd be able to focus on a lot of the fundamentals that would translate to the other cartridges like brass prep and load development. Two pounds of a powder ought to last at least 500 rounds vs about half that for the -06, the bullets are about half to 2/3's the cost... It would make up for the extra $45 bucks spent on dies in a hurry...