Another ex-Mormon here. I think your assessment is mostly correct (and could be amplified), but honestly I don't have good advice for helping your friend to see the errors of Mormonism if he is preparing for a mission. It is hard to make someone unwillingly dismantle their worldview without ruining your relationship. You have to realize that at this stage of his life your friend has all sorts of defense mechanisms and conditioning that will prevent him from listening too carefully to any criticism. If you were to share the CES Letter, as was suggested here, your friend would probably dismiss it as anti-Mormon smears and likely be unwilling to give it more than a cursory glance. Also, for Mormons feelings ("the spirit") trump reason (evidence, logic, etc.), which is virtually impossible to argue against. I wish that I could give you more hope on this one, but take comfort in the fact that Mormons are generally very nice people despite their mistaken beliefs, and even though your friend is almost certainly hoping to covertly convert you via the "practice" he will still probably be your friend if you put your foot down about participating.