Ronan - I have never kept a car in Rome but have dropped several rental cars off right across the street from the train station. There is a multi story parking garage that different rental companies use. Your buddy might check with rental companies and see if they might rent a spot. The other option would be stay a little outside of the historic area at a hotel or B&B with parking and commute. I would expect it to quite a bit cheaper than staying right in the historic area.