The closest thing to steroids is steroids. Test e for 12 weeks will get you 10lbs without even working out, lol.
Just playing but seriously - if he wants to gain and he isnt then he probly isnt missing a key supplement but rather is missing a key in his diet or training. Ask "your friend" () to post his diet/training and training experience as well as stats (height/weight/bf%).
The best way to get over a plateau is to change up diet/training.