I stand by my comment, but shall reword it. There is no such thing as too much power within the limits of the boat (which should be the same as the manufacturers reccomendations) and in sensiable hands.
I deliberatly underengine all the boats for...1) the Sailing Club and the 2) the Sea Scouts because group 1) mostly have no idea what they are doing and group 2)... they only understand stop or go(=full power).
I've owned quite a few boats and, infact, teach lots of peope all about boats every year, so i think i know what i am on about.
A big engine can be as, or even more efficient than a smaller one - for example my last boat would trundle along all day at 25knts, 4000rpm, 30ltrs per hour. My friends boat with a 60hp is flat out to keep up and he eats fuel at about the same rate. The difference is i can make 45knts if i want to (and it's flat enough) and i've bags of power for towing, driving in the rough etc.
With regards to your friends boat i think he should review the set-up. Unless the 150 is substantialy heavier than the (167Kg) Yam 130 then it's either loaded badly, mounted too high / low, not enough offset, or even (as in a previous boat i had) the steering system needs replacing.
I don't know everything, i'm just voicing my opinion and trying to be helpful. I've no more idea of your experience than you have of mine.
I'd regret buying a 5m boat and putting a 45 on it.