You need a 12v regulator that will survive crank.
I'd think that something like the Carnetic units should accomplish what you need.
I have installed my Sirius S50 receiver in my car. Due to the functions of the S50 (it records channels like a TiVo when I'm not around), it is desirable to have it powered all the time, whether or not the car is running.
The S50 (it's base actually) seems picky about power...and when I start the car, the power to the base drops for about a second (power drops to about 9v for a very short period). That loss of power gives the S50 intermittent fits, like dropping the signal randomly, loss of the FM transmitter, etc. These problems are cleared with a shutdown and restart, but I lose the recording buffer when I do that.
The S50 plug converts the 12v from the battery to 5v for the S50. It draws 1.5A at 5v.
I've seen DC to DC power supplies (and other solutions like that), but they are all targeted at PCs which use way more power and seem like overkill for something like this.
I had the battery tested, and it passed.
Sounds more like he needs a 5v DC-DC, to avoid two steps (and efficiency).
I would go for a 5v point-of-load PSU like this one: http://store.mp3car.com/Carnetix_CNX..._p/pwr-016.htm
It supplies up to 3A if used with a little heatsink...
On second thought the S50 is probably to nice to screw around with.
Get a Dc-Dc and treat her right.