My experience
In the U.S. the co-pilot seat is used on many aircraft including the BK, 135, 145 etc. The controls are removed when not being used for pilot training. On the extremely rare occasion when the duals might be installed during an actual HEMS flight, the medic or nurse are instructed to never touch the controls. Also, extra care and additional briefings should be given each time the medic or nurse enter or exit the aircraft cautioning them about bumping the controls.
As for the radios, medcrew are allowed to talk to a scene commander (fire captain, law enforcement etc), or the receiving hospital. They do not talk to ATC under any circumstances. I also found it a great help to have them program the GPS if they knew how and also to tune radios for me if I was busy.
We also had a book containing a picture and description of the many pads and some remote LZ's and they would review that information with us. That was a great help because many times we would fly into a hospital we had not been to and, for example, they had specific approach and departure paths etc. that were required to be used. I also thought that training them on reading the basic IIMC instruction card was invaluable and also some training on deciphering an approach plate for IIMC. Also, med crew can be trained to open up the RFM and read emergency procedure steps as a back up to your actions.
All of this depends on the level of training and commitment of management. I flew in one program that required training in all of these items and I personally found it to be a great help. I also worked with a pilot who absolutely abhorred medcrew from touching anything in the aircraft including radios, books, maps...anything. To bad as they can be of great service, especially in times of heightened adrenaline levels.
cheers