I spent the majority of my nursing career in a helicopter with IFR (instrument flight rule) rated pilots. This has been described by the media as "certified for bad weather." On a handful of occasions, we entered into a cloud layer that was not forecast but this typically happened at night and we had VERY CLEAR procedures for such an event with three key words in mind...Aviate, Navigate, Communicate.... Our service had an administrative order that mandated we fly at a minimum of 500' above the highest known obstacle when in VFR conditions. If we were unable meet that criteria, the VFR flight was cancelled. If we entered IFR conditions inadvertently, the pilot would IMMEDIATELY get eyes on instruments (and we would remind him to do so), we would begin a climb and a slow controlled turn to get out of the conditions - while declaring an emergency. Inadvertent IFR is an emergency.
I believe the rapid descent was absolutely possible - especially if the pilot got into spacial disorientation. The aircraft could've been upside down and diving and the pilot not even know it...