If your TW is well broken in with stock gearing, you won't have any trouble unless your buddy is a very experienced and fast rider. You should be able to do 60-65 easy on asphalt and as fast as you dare in the dirt, unless it's very rough, in which case a bike with longer suspension can soak it up at a higher speed. In fact, the TW will beat out a much heavier bike in some kinds of rocky terrain because it's so light and maneuverable. The real issue is the small tank and range....you will have to carry at least a gallon of extra fuel or subject him to frequent fuel stops, which may not be on the trail.
Sounds like a great trip!