First off a WEEK set up are now enough cycle time. His water now are not ready for piranha, so I would tell him not the get any yet at this time. After 2-3 week check his water parameter, ph, etc...so see if it good and ready for fish. (be safe then sorry)
If your friend want to burn his money and learn the hard way, let him be. He should know piranha are very unpredictable, and 15 piranha would be good in only for couple of month. I can promise you that by the time the piranha up to 6-7" he will lose a few piranha come aggression and territories due from to much piranha in small tank.
150gallon are only good for 6 full grown for life IMO...you can tell him just buy all 15 but sell some as they growing up.