Unfortunately, it was too late, and it exposed the undercarriage of the plane to the rotors ... the helicopter's path was headed at a diagonal into the plane ... the plane seems to be on a slight climb.
I'd say the helicopter.
Most pilots spend their time looking ahead and to their right and left for traffic. It’s very difficult, even in a helicopter, to see an aircraft approaching from the angel those two struck. It’s much like the blind spot in your car on the Interstate.
There is very little right or wrong in VFR operations when it comes to seeing other aircraft. The rule is seen and be seen and that’s about it. There are some altitude rules but they apply mostly to aircraft flying in opposite directions, not two aircraft with one overtaking the other from a blind spot. The helicopter pilot would have to have taken his attention off traffic that might directly in his path and looked back an down to see the approaching fixed wing.
This is a toughie! When you have more than one aircraft in the same general area you have a potential midair collision. Some of them are simply unavoidable based on the type of aircraft and the angle they are closing. It’s even different for various types of fixed wing machines. A high wing monoplane, like the Cessna 172 has a very different field of vision from a low wing monoplane such as the Piper Archer. A biplane has a totally different field from either a high wing or low wing.
The rules for operating on the ground around a helicopter are an indication of how difficult it is for the pilot to see anything behind him, even if only at his 100 degree position. The rules for approaching a fixed wing on the ground and a helicopter on the ground are practically the opposite. You never approach a helicopter from behind, but always want to approach a fixed wing from behind.
The sad fact is that these types of accidents will always be a problem until someone comes up with an avoidance system that doesn’t require equipment in both aircraft.
This is a broad generalization and I would hasten to add the disclaimer that no advice or practice is foolproof in every situation.