a fisheye? really?.....these are best for special effects only. What will he be using this on? his rebel?
I'm sure you know there is a big difference between say a 10-16mm fisheye and a 10-16 wide angle right?
Canon doesn't even make a fisheye for its 1.6, APS-C sensor cameras, like the rebel. They make 2, the
15mm fisheye for around 600 bucks I think and their 8-15mm L Fisheye for 1200.+....but there for 35mm film and full size sensor digital cameras, sure, they will both work on a 1.6x camera but the picture will be crummy.
on 1.6x they can't zoom out far enough to squeeze the entire circular image inside the smaller sensors.

There are 3rd party fisheye lenses like tokina and sigma that will work for the 1.6...those will set you back about 600-700 bucks.
If he is dead-set upon a fisheye I suggest you go to a camera store and rent 1 for a week or weekend . They should have 1 of the tokina or sigmas for the rebel and see if its something he really wants. But if the store suggests the Canon 15mm fisheye, because that might be the only 1 they have, just walk away, their just interested in making a buck because the results will be poor on the 1.6 sensor, and they should tell you that.

what I would suggest, again, if he has to have 1, is that you go the cheapest way possible, and something in the 8mm range, because chances are he will use it for a week and then it will go on the shelf, to be used maybe once a year. you can get these 2 for around 250.00....they will be manual focus, but that's no big deal because with fisheyes you don't focus through the viewfinder anyway....you use the distance scale on the lens

IMHO, he would be better served with a wide angle lens something say 10-16, 12-16 or 10-24..a ton of use out of those...again try the camera store first on a rental....you would be looking at around 600.00 and up but would try to find one refurbished by the manufacture or used. Tokina makes a great wide angle for canon, 11-16mm I believe

but here are 2 for the rebel

http://www.adorama.com/PRO835EOS.html
http://www.adorama.com/RKHD8MC.html