Speaking from experience, you CAN upgrade a few things on them. Often you can swap out (or add) HDDs, video cards, RAM (free slots allowing) PSUs, and/or some things like that. If he looks up his motherboard model, he can find what socket it is and theoretically upgrade the CPU as well.
Dell uses proprietary PSU's, so that's out of the question, but yes, HDD's, vid cards, and RAM. The cases are sometimes reusable, but it's on a case-by-case basis... (lol, pun.)
Sounds like you had my set-up right before I upgraded! I had a P4 as well, and it is an incredible bottleneck. Those processors were never really great for gaming, it turns out. It was a bottleneck even for Counter-Strike: Source, which is an ancient game...
If it were me, you can build a more-than-adequate box using AMD. Intel i7's are yet to be fully utilized by really
any game to date. You can get a very, very decent quadcore (or even a hexacore!) and AM3 motherboard for a very decent price in comparison to Intel's prices.
For example:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103727I hear you can overclock this processor to 4Ghz as well (barring motherboard limitations), and that actually puts it in competition with the i5's and the lower i7 if I recall correctly. However, I'm willing to bet you won't have to overclock anything for some time, and this processor will be more than enough for this game.
If you decide to build your own rig, one thing I suggest: do not go cheap on the powersupply! I've watched custom builds become unstable after a year of use because the builder skimped on the PSU. It may be the appropriate wattage on the box, but chances are, if your using close to the max wattage on a cheap unit, your going to get voltage droop (also known as v-droop). Stick to well known companies, like Antec or Corsair (I'm personally an Antec powersupply fanboy, but that's another story).