If he's got the Boston system, he's got two options:
1. Buy a CANBUS adapter that will let him continue to use the stock amp. Totally plug-and-play. PAC and Scosche (among others) make 'em, they run around $125. Keep in mind this will also retain the stock crossover points (since the amp handles that), so it's not a good option if he wants to throw in a set of components for the front. Good option if you're keeping the stock speakers or throwing some new coaxials in the front doors and/or dash though. Crutchfield sells the PAC C2R-CHY4
2. Buy and run the wiring for an amp bypass kit. Amp is out of the picture. Less money, more work. If you're not changing the speakers, this probably isn't a good option.