Now for my serious response....
It depends. What about his bass is too quiet to him? Is there enough bottom end for their liking? What does the rest of the band think? Have you sat down with him and the band, turned up the bass to his liking, and then ask the band what they think?
At the end of the day it's your job to do what the band says. If they collectively agree the bass is too quiet then you need to make it louder.
Here is how I work with my clients...
-I create a mix without band members present
-I have the band sit down on their own time and come up with a list of changes they ALL agree on
-They all come in to the studio to "mix" the album
Usually, the changes that are made consist of maybe making a vocal louder, making a snare more quiet, adding reverb to a synth...little things. All of the projects I record don't sound a whole hell of a lot different than the finished mix as we are recording it. I make it a point to make decisions up front and get the sounds we want going to tape (or hard disk). I really don't like the "lets record flat and make it sound how we want it later" approach.
When I mix something recorded by someone else I'll usually sit down with the band, listen to any rough mixes they have, figure out their expectations, and then decide if I want to work on it.