Your buddy may be one of the guys that had to have a 454 Casull when they first came out and there was a bunch of used Casull's in the same shops a month or two later.
It seemed like a neat gun to have for me as well until I shot one belonging to a friend of mine. We went to an indoor range, he pulled out his Freedom Arms 454 and told me to fire away. I loaded it up and regretted it. Fired one round and had the rest of the cylinder to go. That was a long time to fire those rounds.
Another friend and I often commented that he offered the Casull to other's to shoot, but we didn't recall him shooting anything other than 45 Colt in it.
If I were you, I'd do as already suggested. Get him started shooting with a .22, then a 357 mag w/ 38spl, then 357mag light, then 357 stout. If he is still interested in moving up let him shoot 44spl, then 44 mag. I'll bet he'll be satisfied w/ the 44mag and a keeper and not want to lay out the cash for the Casull or the 500.
After shooting 454 Casull rounds, my curiosity is satisfied and the urge to own one is long gone.