Sheepdog,
Your friend has a wonderful weapon and I encourage him to practice with it. The 350 will not let him down.
For a 206 gr cast GC Lyman recommended Unique 14gr=1522'/s and Max 18gr=1760'/s. They gave no pressure #s. IMR 4227 29gr=1776 and max 33gr=2004. IMR4198 39gr=2032 and Max 42=2164. IMR3031 41gr=1805 and Max 50gr=2298'/s.
IMR showed 4227 at 38.5 grains (MAX) got 2420 foot seconds. I assume that was with Jacketed. With a 180 Cast LBT FN-GC 29 gr. produced 1921'/s in my 600. 38grains of ReLoder 7 drew very close to 2000'/s.
I have used 17gr Unique behind a plain based LBT WLN 240gr that drew 1552'/s and amazing accuracy. This is a MAX load according to Lyman. This load gets close to the old 35 Winchester and the 38-55. I am told by older wiser men that they are quite capable of harvesting deer sized animals with ease.
13 grains of RED DOT behind that 240 got close to 1400'/s with excellent accuracy. That is 44 Mag pistole' weight and velocity. Again capable of dropping deer. I consider this a Max load for me. (Thank You C.E. Harris.)
With cast bullets I don't recommend striving for jacketed velocities. I think its a waste of powder and it typically produces disappointing accuracy. Again, this attitude is thanks to C.E. Harris, a most reputable contributer. The above loads are much more fun. My 600 at full bore with the 250 gr. round nose will talk me out of practicing fairly quickly. These lighter loads will let me practice the repeat shot over and over with no pain.
I have a lot more load data on the 350 RM in the 600 but I suggest you try these light loads first. When you run out of 206 grain GC slugs use any 357 round nosed, plain based pistole' bullet you have. When you keep velocity down you don't need the gas check. If you want more, email me. I have no secrets and I keep no secrets. What's mine is yours and I hope it helps.
Helice