I think that as long as Johnny was able to do his job correctly he should have been allowed to do so. I think if Roy (or even Johnny) felt that he was not able to work on his friend then their roles would be changed...Roy providing patient care while Johnny handles the base contact and radio report.
Johnny's friend was so badly injured that he needed both paramedics working on him.
After the death I think that if Johnny felt (even Roy or the Captain) that he was not able to work he would have been relieved of duty.
Johnny did spend time with his friend's wife helping her make funeral arrangments.