It's no different to a light fixed wing aircraft. It looks like he flew himself into a classic CFIT.
After crossing the ridge and met with IMC, it appears that the pilot attempted to power his way out. Unfortunately, the aircraft didn't have enough engine performance and by continuing to pull collective pitch, the engine topped out, followed by the rotor RPM decreasing - which is known as "over pitching". At that stage the aircraft stops climbing (effectively the rotor blades are beginning to stall) and directional control in yaw decreases because the tail rotor is geared to the main rotor and suffers in the same way. Game over.
Had he approached the ridge at an acute angle, he could have left an escape route off to one side with known visual cues (because he had just flown past them). I used to teach my students never to fly to a ridge at 90 degrees.
Even in good visibility, if something goes wrong, such as engine failure in a single, or partial power loss in a twin, or even just a lack of sufficient power to make the angle of climb (such as in encountering a down-draught), you have a ninety degree turn to make to escape the rising ground and have to decide which way to go. Approaching a ridge at an acute angle, the escape turn need only be a small one - and you already know which way to turn.