I would suggest something about the infamous three-week stall. It seems like there's a post almost every day from someone who's about three weeks out, frantic because they haven't lost any weight in several days. I really wish more clinics would tell patients about this early stall, because it happens to the majority of us, and most people don't know about it ahead of time so they freak out when it happens.
also, something about how the rate of weight loss is dependent on many factors, some of which you have little to no control over, like genetics, how muscular you are, starting BMI, age, gender. And also not to expect to lose 30+ lbs the first month, because that's not common with people unless they're the size of the people on "My 600 lb Life". Just let them know that if they stick to their clinic's plan, the weight WILL come off. Their ultimate success is dependent on how compliant they are, not their rate of weight loss.