whats the purpose of taking Whey Protein? is it for people who dont get enough protein in their daily meals? or is it for people who want to get massive huge?

now my brother is 5"11 150 lbs and he lean and has muscle but wants to know how much protein he needs to muscle build? he wants to stay a lean mean powerhouse for basketball. should he take 1g of protein per pound? he works out 5 days a week intensely. so if he takes 1g per pound he should take 150g correct? and if he doesnt get enough protein should he take whey protein? if he does take whey protein will it make him fat? he has a healthy diet, enough water for his kidney not to get messedup and exercises daily.

should he take the whey if he doesnt have enough protein?