AFTER A SHORT STOP BTN FLTS, THE BATTERY WAS TOO WEAK TO CRANK THE ENG. THE PLT ELECTED TO HAND-PROP THE ENG WHILE HIS NON-RATED PAX HELD THE BRAKES. HE BRIEFED THE PAX ON THE PROPER PROCEDURE. HE HAD PREVIOUSLY ALLOWED THE PAX TO OPERATE THE BRAKES WHILE TAXIING, & BELIEVED THAT WITH THE BRIEFING, THE PAX WOULD BE ABLE TO OPERATE THE BRAKES DRG THE HAND PROPPING OPN. THE ACFT WAS NOT CHOCKED OR TIED DOWN. THE ENG STARTED IMMEDIATELY WHEN THE PLT PULLED THE PROP THRU. THE PAX RETARDED THE THROTTLE AFTER THE ENG STARTED; HOWEVER, BEFORE THE PLT GOT TO THE COCKPIT, A 'GREAT GUST OF WIND' FROM BEHIND THE ACFT PUSHED IT FWD & IT BEGAN TO ROLL ACROSS THE RAMP. THE PAX WAS UNABLE TO STOP THE ACFT OR STEER IT AWAY FROM OTHER ACFT. SUBSEQUENTLY, IT COLLIDED WITH A PARKED LEARJET & BOTH ACFT WERE SUBSTANTIALLY DMGD. THE BRAKING SYS OF N7151L WAS EXAMINED & RPRTD TO BE FUNCTIONAL. FBO PSNL RPRTD THAT A POWER CART WAS AVAILABLE FOR A 'JUMP' START, AS WELL AS WHEEL CHOCKS. N7151L HAD A CASTERING NOSEWHEEL & WAS STEERED BY USING THE BRAKES.