Yes, they are definately accurate as far as lever actions go. Mine is easily capable of MOA with quite a few loads, and I can't count the loads that have shot under 2".
If accuracy is your number one goal, then I'd suggest a load with Hornady 140g XTP-HP's in both the 38's, and 357's. Out of the dozens of different bullet styles, and weights I've tried over the years, the 140g XTP-HP is hands down the most consistantly accurate in both rifles, and revolvers. I can about garantee you that if you'll load these with just about any powder from minum to maximum velocity you'll be pleased with the accuracy.
I'm going out on a limb here, but in my opinion, if a 357 magnum(rifle, or revolver) won't shoot this bullet well, it likely won't shoot any bullet worth a hoot.
I'll second Jerry's words on the 140 XTP-HP, they are VERY accurate, and easy to shoot well with a variety of loads. You can load from mild to wild and still get good expansion, they were designed to be used over a wide velocity range. My guns also like the Remington 158's you can buy in bulk from Midway, I shoot several thousand a year. But the 140 weight is a wonderful compromise between the too-light 125's and the standard 158's. Your friend's gun will quickly tell you what it likes.
As for 38's, some Marlins don't feed both lengths reliably, try some before you load a bunch and see.
I am pleased with the accuracy of my 357, although, with my old eyes, shaky hands, and iron sights, I don't get 1" groups. What surprised me was how much bullets drop between 50 and 100 yards. The handloads I was shooting dropped about 8-10". I've loaded some new ones with Lil Gun that should be higher in velocity. I'm anxious to see if they shoot flatter.
You should tell your friend to watch for a carbon build up if he shoots much 38 and it will prevent 357's from being chambered if not kept clean. This can be avoided by loading the light loads in 357 cases..
My rule of thumb for loading light is to add a grain of powder to the 38 data, for use in 357 cases. That should alleviate any worries about using too little powder, and either getting a KaBoom or a bullet stuck in the bore. Both suck, one a lot worse than the other.
Enjoy that rifle, I don't think there's anything that's more fun, and useful over a wide range of applications, than a 357 Marlin!
I load 15 gr. Lil'Gun under the 180 gr. Rem SJHP or 18 gr of the same powder under the 158 gr. Hornady XTP (either the FP or HP) or Speer Gold Dot. In .38 Special I'm just plinking and 5 gr. Unique under any good 158 gr. bullet that feeds well will suit.
Jerry has had a lot more experience than I so if he says something will work, go with it.