Steve 76,
Any roll, yaw or pitch would be due to rotor rpm droop, as there is no other mechanism for the disturbance. How low did your friend let the rotor go? The normal tendency at the 0 to 40 knot range for all US/UK helos is to require right and forward stick to keep the aircraft level and accelerating. Rotor droop with the right stick in (its about 1/2 to 1 inch on most helos) will result in a left roll and a pitch up, both slight. If the rotor is kept within about 5% droop or so, the afcs will handle it and the pilot will not need to correct. If the rpm goes down a bunch more, it would be more noticable. I certainly would not call any handling quality of the S-76 in this regime "vicious".
I have seen documented evidence of one case where an operator got the rotor down to very very low numbers, in flight, and flew home. Very very means way below 60% Nr. I am not sure how many helicopters are that kind to their occupants, but I'll bet not many. So take care what you call my baby!!
