I've done it many times with experienced stokers, that is, 100 miles for a first ride. You will have to get the fit right and be careful about the cadence. Guessing that you are doing the Leukemia rides, you should be somewhat experienced and fit cyclists by the time you get to Tahoe, ready to do a hilly century around the lake.

The other thing that you will have to be careful about is the fit on the back. If she is tall and the center-to-center bottom bracket spacing is short, her positions will be limited by the space you take up in front of her. The more upright position puts more weight on the saddle, and can tear up bottoms in short order. She should use her preferred saddle, and an appropriate lubricant where she might chafe.

Without any significant experience with heating rims / brake fade on a bike (lots on motor vehicles), I can't comment on that, save to say the problem is usually steep descents with sharp curves that require slow speeds and consequent high energy inputs into the braking system with speed between the corners, and that descents that lack sharp curves and have generous runouts at the bottom are not a problem--air resistance will eventually gobble up all the speed.