Single pilot NVG EMS is fine...depending on the training, the machine, and the operational limitations.
Training has been mentioned already. Get a proper NVG conversion, then learn from someone doing the same role in the same machine (similiar if same not possible).
The machine needs more than cockpit compatibility (which is usually restricted to cockpit lighting compatibility). The switches must be identifiable with NVGs on. The immediate actions cannot be delayed due to the NVGs. The handling of the helicopter must remain acceptable with the reduced visual cues. These issues cannot be taken lightly. Employ a test pilot to evaluate your helicopter in that expanded role.
The operational limitations have been mentioned already. The weather is definitely a concern. I support lower weather minima for unaided night flight over aided. When on scene many operators are hesitant to use white light. Don't be. EMS is not a covert operation. Light up the area and see the wires.
While the safety concerns are valid, the increased service to the customer is invaluable. Lives will be saved.