THE INSTRUCTOR (CFI) WAS PROVIDING A MOUNTAIN (MTN) FLYING CHECK-OUT TO A PVT PLT. WITNESSES RPRTD THAT BEFORE THE ACFT CRASHED, IT WAS FLYING 'VERY LOW' UP THE MIDDLE OF A VALLEY TOWARD A 12,700' MTN PASS. IT THEN ENTERED A TURN & DISAPPEARED FROM THEIR VIEW BEFORE CRASHING. IMPACT OCCURRED AT AN ELEV OF ABOUT 11,200'. THE PVT PLT WAS FATALLY INJURED; THE CFI & PASSENGER RCVD SERIOUS INJURIES. THE CFI (A 95 LB WOMAN) STATED THAT THE PVT PLT WAS BIGGER THAN HER & THAT HE KEPT OVERPOWERING HER. SHE SAID HE DID NOT FIGHT HER FOR THE CTLS, BUT WOULD NOT FOLLOW HER ADVICE TO KEEP THE NOSE DOWN TO MAINTAIN A SAFE SPEED, & THAT HE WOULD NOT FLY ON THE WINDWARD SIDE OF VALLEYS. AFTER SHE DIVERTED HER ATTENTION TO LOOK AT A MAP, SHE LOOKED UP & NOTED THEY WERE IN A VALLEY WITH RISING TERRAIN ON ALL SIDES. THE PVT PLT RELINQUISHED THE CTLS & THE CFI ATMTD TO REVERSE DIRECTION, BUT THE ACFT IMPACTED THE SIDE OF THE VALLEY.