I think Dad should decide if he wants an SLR (in which case you are buying into either the Canon or Nikon system) or a digital camera, such as the Coolpix. I don't see how you can compare these two cameras.
The Rebel XT is a digital SLR. Nikon's digital SLR is the D70 (about to be the D70s).The Coolpix 8800 isn't an SLR. Canon's closest alternative is probably a Canon PowerShot G6.
In the Overall Conclusion, in its review of the 8800, dpreview.com summarizes the difference between an digital SLR and a digital camera
read the conclusion
I've used the Canon PowerShot G3 (which I recommended that my Mom purchase), and I have a Nikon D70. I already had a Nikon film SLR, so the decision to buy the Nikon vs. the Canon was already made for
me. I think the G3 is a great camera, but the shutter lag drives
me nuts. That said, I already owned a speedlight and 4 Nikon lenses, so I had a lot of the stuff you'd need to buy to make the SLR as versatile as the 8800 from the get go.