I don’t know why the co-pilot would want to nose-dive into the ground, but maybe check that he/she has not been accidentally given control of the aircraft.
Also, a check of your bindings, to make sure that there is nothing in there that might mess up. For example, even aircraft without a fuel mixture control can get a bit upset if your mixture lever is set to zero. The start of the flight might be OK, until you either touch the lever or Esc and resume, as that will set the virtual control/axis to zero if that’s where your control lever is.