Look for 2 books written by Horst; Training For Climbing and How to Climb 5.12. The books should answer most of your questions. My suggestions:
Only hypertrophy muscles that will assist in climbing (a big chest and legs is excess weight)
Strengthen grip for all possible hold types (look up hit strips at nicros.com, allows you to work open hand, crimp, all pocket combinations and pinch grips)
Core strength is critical, especially on overhangs
What type of climbing does your friend perform? Bouldering? Sport? Trad? By the way, Horst’s books are available on nicros and you should be able to find several articles by him that pertain to improving climbing performance.
Let me know if you have any specific questions.