Hi Adam, welcome to the forum.
It has been my experience that no matter how dark a picture you paint, first time pass users are by nature a very optimistic bunch, the poor dears . So like Paul, I would suggest you go over in minute detail back-up plan B, C and D and then explain how 'priority lists' and/or thunderstorms a thousand miles away can wreck a whole operation. With my first timers I emphasize the 'space available' point and hammer it home by providing hotel and rental car 800 numbers and suggest putting a night's provisions in their carry-on. (Trust me, nothing explains bumping like pj's and an extra shirt in the carry-on ) I recommend having the 'will we split up' decision made before starting for the airport and advise checking in e-a-r-l-y, traveling light and being flexible and patient. Finally I put my number on speed dial and remind everyone it's always nice to bring a little something home for me.

Edited for spelling and stuff

Keep'em flying