First off: Amazon is out of TftNA books. The new edition will hit the Patagonia warehouse in Reno soon so your friend can order from Pat or Amazon and will get the latest version.
Congratulations for stepping up. This will really help your skiing.
Doing long aerobic base workouts on the Skierg is possible but quite boring. Roller skiing is better fr=or this. Most people use them in an interval protocol that focuses either on maximum power (3-6 seconds with 3-5 min rest), anaerobic endurance (30-60 seconds with 3-4 min recovery) or aerobic endurance intervals of 3-6 min of maximum sustainable effort with 2 min recovery. A mix of all these can be beneficial depending on what your goals are. The max Power will really only be of much value to sprinters (XC and Skimo). The Anaerobic endurance will again be of most use to sprinters. One to two of the aerobic endurance sessions per week will really pay off in XC and have a significant effect even in skimo. Look for an article in the next 2 weeks describing how we use this machine for interval training. I’m working on it now and it will be posted soon.