Try www.caraudiosecurity.co.uk for purchases.
When dealing with car stereo's you need power, if he is gonna run it of his standard head unit, his speakers will lack power, they are also not designed to have much power running to them.
Sounds to me that if he wants his music to sound good he should invest in some decent speakers at the front to replace the original ones. An amp in the boot supplying power to them, and if he wants a sub, he won't need 6X9's because the sub will supply bass. If he is gonna have sub, make sure his parcel shelf isn't gonna rattle around from the bass.
Any more questions, just ask.