First to address the original question. This sounds like it's not a terribly photo-geeky purchase and rather the 10 mp number is sort of arbitrarily being thrown out there as what the purchaser wants to have because that's what they want to have. That's all well and good but if this person is going to have to print at sizes greater than 8x10 to notice any difference between even a decent 6 mp SLR and a 10 mp SLR.
Second, I personally cannot stand any Canon Rebel camera, never have liked em. To me they're overly small plasticky toys with bad pentamirror viewfinders and no spot metering. But what displeases a photo geek like me probably is exactly what is good for a female (smaller hands) non-enthusiast person like your friend. In that case a Rebel is fine. Might as well snag an XT while they're still on shelves because it's a better deal and the 16% improvement in same-lens resolution you get from the XTi needs larger format prints to be seen.
Now to sherwood: you are correct that if you compared two theoretical sensors that were completely identical save for the number of pixels, the pixels on the sensor with less pixels would be more sensitive and thus that sensor would have a higher signal-to-noise ratio.
BUT.
All sensors are not created alike, and photosite size alone does not determine the light sensitivity and signal-to-noise ratio of the sensor. A lot of ground can be made up with the materials of the sensor itself, design changes to other bits of hardware, different microlenses over the photosites, etc. Point being, in tests the XTi has functionally identical noise performance to the XT.
From
dpreview (400D = XTi and 350D = XT for anyone who can't follow along):
So yeah, with the XTi you get no increase in noise, no increase in useful features that a casual snapshooter who just wants an SLR because it makes prettier-looking pictures than a compact, and 16% more resolution out of the same lens were it mounted on the XT.
If this were a more "serious" inquiry then I would lean much more heavily towards the Nikon D80 (which, yellowbastard, is not Nikon's Rebel...that's the D40 and D50), Pentax K10D, or Canon 30D (even though it's only 8 mp, it's closer than 16% away from the K10D and D80).