fuel cut is not at a specific psi on the 7mgte, it uses a karman vortex air flow meter which fuel cut is then derived by frequency, generally it will be at 1500hz or for easier understanding at a certain air flow it will cut
fuel cut will change with everyones car, with a lot of air mods (intercooler, turbo etc) it will be much lower than stock
for him to be boosting at 15psi i would assume he already has a fcd and as i said previous i hope he has the fuel to support it as the stock ecu can't supply the fuel for that
if you do a diagnostic you should find code 34 if you are hitting fuel cut
abnormal turbocharger pressure
