THE FLIGHT INSTRUCTOR (CFI) AND DUAL STUDENT DEPARTED IN A HELICOPTER AT NIGHT ON A COMBINATION NIGHT INSTRUCTIONAL/VFR FERRY FLIGHT. THE PILOTS WERE FOLLOWING AN INTERSTATE HIGHWAY, WHEN THEY ENCOUNTERED AN AREA OF REDUCED VISIBILITY. THE CFI TOOK CONTROL OF THE HELICOPTER AND MADE A 180-DEGREE TURN. HE REPORTED THAT HE ENTERED A DESCENT TO IMPROVE VISUAL REFERENCE TO THE GROUND AND SLOWED THE HELICOPTER TO ABOUT 40 KNOTS. HOWEVER, THE HELICOPTER CONTACTED THE SURFACE OF A RICE PADDY, SLID IN A LEVEL ATTITUDE UNTIL IT ENCOUNTERED UNEVEN TERRAIN, THEN ROLLED OVER.