Shutter noise is still the same, and what goes up (the mirror) must come down- and must go up in the first place, so you'll have the same noise, just spaced out some and likely not quiet enough for a set.
It's possible that just taking shots when nobody is speaking might be good enough for some sound editing if it's ok with the post-production crew. But the best bet on a low budget is just to do a few extra takes for the photographer.
There is a sound reduction feature for Live View, at least on the 40D. However, there still is some noise left from the shutter. On Live View, the mirror has already been flipped out of the way, so the only sound left is from the shutter moving. In the 40D (Live View mode), you can reduce noise as follows:
-Mode 1: Mirror stays locked and out of the way, and the shutter's noise is reduced, but it allows for continuous shooting (burst).
-Mode 2: This mode reduces shutter noise further (only a slight click is heard), but continuous shooting is not possible.
I don't know if he can do the same with his XSi. Maybe he should read the manual further?