I travel a lot due to work, and what I do is search for free wi-fi connections and plan where I go before going out.
Make a snapshot of my iPhone screen by holding down the sleep/wake buttom + the home button.
Sometimes I am forced to activate the roaming and it hurts my wallet.
If he is going to drive a lot, a GPS unit may be the best way to go.
Sadly the iPhone roaming charges are huge.
I am not sure if unlocking and using a local pre paid SIM with data plan will be better than investing in the GPS.
TomTom is supposed to have better maps in Europe than Garmin, but Garmin has better keyword search alternatives, TomTom forces you to search by street name and number (you need to know the address) or by their loade POI, Garmin also allows you to do a keyword search.