.357 is plenty for varmints, two or four legged. Also smallish game animals. It's got as much power, more actually, as .30 Carbine, which is used by many around here for hogs. Plus more and better defensive loads are available than for .30 carbine. For the two legged variety of varmint, it has the advantage of being a cowboy gun not an evil assault weapon, which in some jurisdictions is a consideration, yet it's quite effective as a home defense weapon.
You can shoot .38 special for practice. That allows for cost savings, and since it's a less powerful round, one not as likely to develop bad shooting habits, in spite of shooing a lot of rounds.
Expensive? Well what ammo isn't in these days Obama? But a quick check found re-manufactured .38 spl at $65 for 250 rounds. Less per round for larger lots.
But for comparison from the same site, and for the same brand (Fiocchi) and bullet type (FMJ) I found 50 round of .38 Spl for 18.57 and .357 for 18.97. Oh, 40 cents difference for 50 rounds. or .8 cent per round.
However I'll concede the "hard to find" virtually all of that ammo is back ordered on the site. But then again, so is the .45 ACP I ordered a couple of months ago.
Going to the range tomorrow to see what the .38 spcl Hydra Shoks do out of my Winchester 92 ‘Red Rider’ (it has the ring on the left side of the receiver). It cycles .357 jhp ammo nicely. With the tube full, that rifle has a great balance to it.
Just that it's on the low end of the scale for a rifle round and if he wants it for say whitetails he'd be better off with a 30-30 or a .308 and the occasional varmint.
If he wants it for plinking and home defense it would be fine.
I would suggest that anyone that wants to shoot on a regular basis, get into reloading, which I think you already do.
A nice rifle load for the .357 mag rifle is 125 gr. SP over 22 gr. of H110 or 158 gr. SP over 16.7 gr H110.