DozyWannabe: No, Not an excuse, I am trying to point out to you what was likely to happen in the real world.
Second No: The fact that the radar controller agreed to the letdown does not imply he should be aware of the position.
The aircraft was still some way out, and only the pilot could make the decision about descending VMC,. The controller would not know what meteorological conditions the pilot was experiencing at the altitude he was at the time, or those conditions in the area the pilot was intended to let down in.
Let me make the point quite clear.
If I were to issue a VMC descent clearance requested by a pilot, I do not necessarily expect him to maintain his flight planned track. He may deviate from it to maintain VMC.
He is still on an IFR flight plan, so my responsibilty as the controller is to ensure his separation from other traffic is maintained.
In the case we are discussing, there was no other traffic.