A STUDENT PLT & PASSENGER TOOK OFF AT NIGHT IN IMC CONDITIONS WITH LOW CEILINGS, FOG & RAIN. A WITNESS, WHO SAW THE ACFT FLY OVERHEAD, VERIFIED THE CEILING WAS VERY LOW & THAT RAIN WAS FALLING, SOMETIMES HEAVILY. SUBSEQUENTLY, THE ACFT CRASHED ABOUT 2 MI FROM THE ARPT AS IT FLEW INTO TREES ON ABOUT 5 DEG RISING TERRAIN. THE PASSENGER WAS FATALLY INJURED, BUT THE PLT WALKED TO A ROADWAY & WAS HELPED. THE ACFT WAS OWNED BY THE STUDENT PLT'S FATHER, WHO SAID HE TOOK IT WITHOUT PERMISSION. BEFORE FLYING, BOTH OCCUPANTS OF THE ACFT WERE SEEN CONSUMING ALCOHOLIC BEVERAGES. FOUR HRS AFTER THE ACDNT, BLOOD WAS DRAWN FROM THE PLT. A TOXICOLOGY CHECK SHOWED IT HAD AN ALCOHOL LEVEL OF 0.075% (75 MG/DL). A CHECK OF THE PASSENGER'S BLOOD SHOWED AN ALCOHOL LEVEL OF 0.085% (85 MG/DL). POLICE RPRTD THE PASSENGER WAS NOT WEARING A SEAT BELT.