I'm noticing that no one is clearly answering this straight foreward...
Your friend has told you that you need 0 gauge wire right? Because it helps more current flow to your amp then say 4 or 8 gaue would. This is correct. A bigger wire helps more current flow. Easy concept. Now, where from the very start is that power that flows through that 0 gauge wire that is going to your amp coming from? Your alternator.
That power flows from the alternator, to the battery, through your 0 gauge wire to your amps. Why not also upgrade that wire (which is some cheap, stock wire..you wouldn't use cheap, stock from your battery to your amp, would you?) from your alternator to your battery to allow more current to flow? Your alternator generates power...but going through small, stock wire you aren't going to get all of the juice your alternator can get. Make sense?