What you need to do is ask your friend how the house is actually wired, otherwise you won't know what you're capable of doing.
if there is only cat5 at the volume control locations there are kind of two possibilities.
First possibility benefits you if you are cheap and con't care much about zone audio: electrician was an idiot and wired the speakers for volume controls with cat5 instead of speaker wire. In this scenario, cat5 goes through VC location to speakers. USe the cat5 as speaker wire. Not ideal, but you're stuck with it, and it's the cheap way.
Second possibility: whoever wired the house planned a more complex system using keypads, which control a whole-house system which is all home run, apparently (given you only found cat5 at VC locations) NOT looped through the VC location. If the cat5 terminates at the keypad location and the speaker wire is elsewhere, you basically have to go with a whole-house system using keypads to control each zone since you cannot use volume controls unless you can locate the speaker wire in the walls and cut in volume controls there.
But the permutations of volume controls are endless depending on what kind of volume control, how it is controlled by the system(if at all), how many speakers are being driven off a single amp (impedance matching VCs), how they're setup, etc. Without knowing how the house is wired, or what system is being designed for it, not really much specifics anyone can help you with. Suggest you hire a pro, or figure out all the details if you want to do DIY yourself.