This is one of those situations where either one would do the job fine and you won't see a big difference which ever one you chose. But here is my take:
The XL has the 16 pipes which comes in handy in some of the newer games. But you will doubtfully be able to come within 100mhz (with overclocking) of the Pro because the XL doesn't overclock well (atleast the one I had didn't). And I have found that a lot of the games out their can be very CPU intensive and seem to benefit more with a higher core clock. So in this case the Pro is the better way to go. So what it really comes down to is "What games will he be playing with it?" If he is playing UT2004, for example, he'd be better off with the Pro. That game does'nt need more than 12 pipes to play flawlessly but really benefits from higher core speeds. But if he's playing Half Life 2 the extra pipes will probobly do more than the extra core speed. Another thing to consider is at what resolution is he playing his games. If it is 1280x1024 or lower, 12 pipes is all he needs and the extra core speed will be the greater asset. But resolutions higher than that and those extra pipelines will really come in handy.
I hope this helps, Eric.