Pete - haven't you pointed him to the 2 most dangerous roads in the area
I don't believe so. I ride on them regularly... And there are far more dangerous roads in Rome, unfortunately.
The Colombo is not too bad, I regularly ride it after work (ergo, rush hour) for my training rides. The Tiberina is actually quite a nice road, with limited traffic, by Rome standards.
There are, of course, plenty of other roads, but you usually have to go out on one of the 'consolari' (consular) roads to get to them.
For the record, the most dangerous road in the Rome area, by far, is the Aurelia. I believe that for some time it held the Italian record in terms of deaths per km. They actually built a trauma hospital there for that reason.
If he is then willing to take a train out of town and then ride, then there are fantastic training routes with wonderful roads and little traffic. But that becomes a full day affair.