I gotta throw my .02 in!
Your original question was would .358 hardcast @ 1800 fps properly plased by a competent young shooter do the job on a cow elk. I believe it would do it quite well!!!

Would it be my 1st choice? You didn't ask that question (But the answer is Nope). BUT, I have no doubt that in your son's capable & experienced hands it will work & might be his best choice.

Vance in AK.

Matthew 6:33
"But seek first the kingdom of God and His righteousness, and all these things shall be added to you."