BEFORE TAKING OFF FM THE UNCTLD ARPT, THE FLT OBTAINED A WX BRIEFING, FILED AN IFR FLT PLAN & RCVD AN INSTRUMENT CLNC BY TELEPHONE WITH A VOID TIME OF 15 MIN LATER. A WITNESS ESTIMATED THE ARPT WX WAS 200' OVC, VIS 1 TO 5 MI WITH FOG, AND STONE MTN (NORTH OF THE ARPT) WAS OBSCURED. THE ACFT DEPD FM RWY 25 WITH 4 RATED PLTS ABOARD. THE OWNER (A NON-INSTRUMENT RATED PLT) WAS IN THE LEFT FNT SEAT & A COMMERCIAL RATED INSTRUCTOR PLT (PIC/CFI/CFII) WAS IN THE RGT FNT SEAT. SHORTLY AFTER TAKEOFF, THE ACFT CRASHED INTO STONE MTN, APRX 2.5 MI FROM THE ARPT, ON A HDG OF ABT 310 DEG. AN EXAM REVEALED THE THROWOVER YOKE WAS TO THE LEFT. LOGS SHOWED THE OWNER HAD BEEN RCVG INSTRUMENT INSTRUCTION FM THE INSTRUCTOR IN THE RGT FNT SEAT. THE INSTRUMENT DEP PROC WAS TO CLIMB VISUALLY OVR ARPT TO 2800', THEN CONT CLIMB VIA DRCT LONG HOLLOW NON-DRCTNL BCN HOLDING PATTERN TO 5000' BEFORE PROCEEDING ON COURSE. ETHANOL WAS DETECTED IN THE RGT FNT SEAT OCCUPANT'S BLOOD; HOWEVER, THERE WAS EVIDENCE OF DECOMPOSITION IN THE SAMPLE.