Commonsense alone instructs us that if you are flying below terrain in a valley....Radar is not going to see you even if you have a Transponder operating.
The Controller knew he was going to lose Radar Contact with the aircraft due to the altitude it was maintaining and the route it was following.
ATC is not required to provide additional services to VFR traffic when their Control Zone is IFR....their job is to provide services to IFR Traffic.
The one exception is when they have a Special VFR flight operating INSIDE their Control Zone under a Special VFR Clearance.
As to their radio transmissions to the Accident Aircraft....they could be ignored by the Pilot if he was becoming overloaded by the deteriorating weather.
What is that old rule....."Aviate, Navigate, Communicate"....in that sequence.