The above statement is completely untrue---please forgive me I do not like contradicting a PB member such as Beer30 as he is a very knowledgeable person. However, I shake my head when this type of misinformation is quoted to the public.
Both the HP600 and HP700 are affected by altitude changes [1000ft+] and [arizona heat 100+] and to say that the motor produces the same HP all the time is untrue. Why! because there is no way the ECM can compensate for such changes and that is not muddying the factual waters when you drive the combination in those types of conditions you will simply not have the power delivery in 105 degree heat at lake Tahoe and that will be perfectly clear when you push the throttles forward.
If you need a second opinion, Team Mercury came out to Havasu in July to install two HP700 motors and said the exact same thing when conditions were very hot ie...100+ motors aren't gonna make the power for the top end we were looking for in those conditions.
Initially, the HP600 had some code problems with the motor wherein the water from the intercooler was being dumped directly into the exhaust and causing problems ie... creating to much back pressure and setting off codes. This was remedied by dumping the water out of the side versus through the exhaust.Once this was corrected the motor runs strong and pulls great everyone was happy.
If your friend currently has a HP525 in his boat--- he can explore the Whipple option [AX200] for that motor provided it is not under warranty. The Whipple option is not that expensive bolts on in a rather simple procedure and produces great horsepower depending on the boost level you choose for your application.
If you seek a complete motor swap going from a HP525 to a HP600 is not a prudent idea the HP gain is not that dramatic and only the delivery of the power ie...throttle response is the only real difference albeit it is quite noticeable--- but overall in my opinion I would simply Whipple the 525 provided it was still rather fresh ie...low hours.
More facts are needed in your scenario as your question is overly simplified but I hope this assists your friend in his/her decision making and/or gets you pointed in the right direction budget wise.