Its absolute bollocks to say he needs 500 watts! But you do need lots more watts than guitars, 2-3 times as much, so if the guitars are using 100watts the the bass needs 2-300 watts. Thus is cos guitars have a narrower frequency band to amplify and distorted guitars amps usually sound good whereas distorted bass amps usually sound bad and f*** your speakers. Speaker efficiency is also an important consideration.
£300 is not a lot of cash for a gigging rig so its best to lurk around the classified ads on here and your local Loot, Friday Ad etc.