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Well he'd need a new receiver, and would car speakers work fine?
Depending on his goals, that $250 might not even be enough for a receiver. Read through my previous posts on this thread - unlike home systems where you can have a decent speaker and t-amp and then maybe an old iPod for that $250, a car is much more complicated. For one, you're sitting off-center, unless you have a McLaren F1 (in which case I don't think you'd get stuck in traffic using it on a daily basis to bother with a sound system); second, the tweeters and midwoofer aren't mounted close together in a purpose-built MDF enclosure with a tuned port.
Car speakers will work fine, and as I've previously posted, it is highly preferable you spend the extra cash on car audio drivers over cheaper home audio raw drivers for a variety of reasons. For starters, they have relatively shallower basket depth and smaller magnets for mounting in doors, since home audio speakers are designed for a custom enclosure, they don't really worry about this; also car speakers are designed for essentially free-air mounting (since your doors have some space between the rubber and the window glass).
However, the $250 is just about the price of entry-level speaker systems and an amplifier from the same series from most brands. You can get better performance at around that price from, say, Swans; however you aren't going to get quality speakers any cheaper. There's always the raw driver option, but you'd have to DIY, otherwise your professional custom installation costs would probably go through the roof.
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However, if you manage your expectations well enough, you
might get a good enough system for around $500. As long as you don't expect for it to sound like a home system where you can have the vocals dead center on the dash and everything else on the soundstage laid out as they would be at home, you can get a decent receiver that doesn't have the processing tools necessary (like 6-way crossover networks and time alignment that necessitates one amplifier channel for each of them, or actually just five since the last two are subwoofer outputs) for this.
Receiver : http://www.crutchfield.com/p_500IDA305S/Alpine-iDA-X305S.html?tp=5684
4channel amplifier : http://www.crutchfield.com/p_236RT604/MTX-Road-Thunder-RT604.html?tp=35782
Compnent speakers : http://www.crutchfield.com/p_107DXI6500/Polk-Audio-DXi6500.html?tp=106
NOTE : Confirm the right size for your car; even the mounting diameter isn't always the correct size, since mounting depth may necessitate some fabrication
Subwoofer : http://www.crutchfield.com/p_107DB840/Polk-Audio-db840.html?tp=111
Dynamat : http://www.crutchfield.com/p_15410425/Dynamat-Xtreme-Wedge-Pack.html?tp=2277
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Personally though car audio is the more frustrating audiophile project. It's exciting in so far as there's the challenge to get it to sound "right" in a car, however past all that it may be too difficult considering the costs involved. You can blow $1,000 on gear, it comes nowhere near what you have at home and that probably cost less, then it gets worse when you run into a guy who spent less than that, but did fabrication or at least installed everything properly, and it sounds a heck of a lot better because he was able to get around the inherent problems of car installs. If however you totally just don't aim for it to sound like your home system then the improvement from adding a sub or adding Dynamat along with new fron speakers might mean a lot already.
I can always put some Dynamat in it though, also, I'll probably have his truck later (And can DIY the hell out of out then)... 
