If the rotor RPM is already low or decaying, it’s certainly possible. My own guess as to what happened is that the pilot got disoriented, and helicopters have a way of getting in to unrecoverable attitudes pretty quickly when the pilot is disoriented. It’s possible during that disorientation that the pilot allowed the helicopter to get in to a low rotor RPM situation, though without firsthand knowledge of an S-76, I can’t say for sure if that’s what caused the descent or if it was simply a matter of not knowing which way was up.