If you have a base model it should be no problem to update the hu with a ipod compatible.
Even if you have an infinity system that's no problem.

He should just replace the hu and use the internal amps of the hu.
The easiest way to add a subwoofer is to use a subwooferbox with an integrated amp that is powered by the sub chinch out of the hu. So the hu needs a sub chinch out with an integrated crossover. This is very common and even low to mid class hu's have that feature.

I've been using the stock speakers with another amp which had 70 watt RMS @ 4 ohm per channel for a little time withhout any problems. So the stock speakers can handle a little more power without problems.