I had a similar situation with a 30 hour student I took on from another school. I was not in the helicopter as the exercise was a solo one practising basic skills a few miles form the airfield.
Previous school instructors always turned the gov off on the ground and when I was not in the helicopter the student had reverted to type and turned it off whilst running up. You all know where this is going - well he managed to take off with out the gov on and shortly after take off realized his mistake and flicked it on. No problem he thought and off he went - some 5 minutes later he realized the engine and rotor needle was no where to be seen and after doing some trouble shooting he slowly retarded the throttle and then all of a sudden the gov pulled the RPM back to the normal green range. Student believing all was now ok, continued the exercise for a further 35 minutes before returning to the airfield.
Thankfully I have a no blame culture and open approach to all mistakes so he fully explained what happened. Whilst there are many lessons to be learn't here the I grounded the helicopter immediately and had it check - 3 weeks later and $36000 out of my pocket (I also own the heli) found very little wrong and is a testament to how these little machines are made. Worst thing was the helicopter was brand new and had 98 hours on the clock at the time of overspeed.
Lessons learn't - gov stops governing at 116%, never assume that just because a student has over 30 hours with another school that he know the very basics of pre-lift and post lift checks, always brief on the what to do if any unusual situation occurs durning any part of the solo exercise (done before every solo flight until they are responsible for themselves.
Thankfully, no one hurt and a very valuable lesson - student now has 500 hours and owns his own Jet Ranger which we also use for some commercial work.