| Do your buddy a favor and have him check out Rockabillybass.com. Tons of good info about amplifying a double bass in a loud rock setting over there. Many of the members are professional bassists in Rockabilly, Country, and Psychobilly bands, etc.
The pickup he most likely has is a Fishman BP-100, which is a horrible piece of crap. He's going to have a very hard time getting a usable sound out of that thing.
He will need an external preamp for starters (design for acoustic double bass) , because most bass amps are designed for magnetic bass guitar pickups and have low-impedance inputs. A piezo pickup plugged straight into a regular bass amp will have no low-end or volume and sound awefull.
Feedback is a big issue with amplifiying a double bass also and there are many ways to deal with that issue.
The best thing to do is check out the Rockabillybass.com forum and read, read, read, and invest in a decent pickup and preamp. There are many bass amps that will work OK, lots of info on amps over on the forum also. | 
