I find the built in voice dialer to be a problem while driving. You have to pickup the phone and find/tap on the icon to start it. Then for me, the phone often doesn't understand me and prompts to make a selection via an on-screen pop-up that I have to read and tap, especially when a contact contains multiple phone numbers (for home, mobile, work). This is not welcome while I'm driving.
On the Incredible or the Droid, if you are using a standard BT headset there are three possible work arounds...
Voice Dialer HF (DFXmobile)
Choice Dialer
http://www.pwnwithyourphone.com/voice-confirming-dialer
You still have to handle the phone to start them... and due to Android limits, they use the phone's mic and speaker until the call is connected.
The other options...
1. Switch to a Moto Devour, as Moto has licensed and included Nuance VSuite to do the dialing (similar to BB, LG and other non-Android phones),
2. try to find and purchase a BT Headset that maintains a phonebook and does the dialing for you, (like the BlueAnt Q1
BlueAnt Q1 Voice Controlled Headset - BlueAnt Wireless which holds up to 8 numebers to "speed dial")
or
3. dump android until Google gets off its collective rear end and fixes ->
Issue 1181 - android - [VoiceDial] Voice confirmation for Voice Dialer - Project Hosting on Google Code but since that has been open since Nov 2008, I doubt it will be any time soon
Of course, if he has an IN CAR SYSTEM (like MS Sync in Fords) that does the voice recognition and dialing, then its a non-issue.