No one can help you without knowing how the current props are working for your friend. What is his top speed? What are the maximum rpm's achieved at wide open throttle (WOT)?
We'll assume the 13 3/4" props are 4 blade OT's and the 14 3/4" props are probably something like the Turbo Lightning or TXP 3 blade. Although that seems odd since there are no left hand Lightnings or TXPs.
We'll further assume the boat needs tail lift since he's running OT's although there are some bow lifting 4 blade props but not many with an OT hub.