I have seen "The Secret," which is severely lacking in many areas of NLP Outcome specification (see attached). It has become quite popular, and has sold over a million copies to the same "get rich quick" (without any work) crowd who go for MLM and pyramid schemes--the last one was called "Airplane" and was framed as "believing in abundance," when it was actually a blatant rip-off.
If your guest will basically be presenting "the secret," (which is hardly a secret) you will be perpetuating a half-truth, and that is not a favor to your listeners. The "law of attraction" implies that things will simply come to people without effort if they vividly imagine getting them--and in "the Secret" this is exemplified by the boy who is wishing for a bicycle, and then his uncle (I think) gives it to him, without the boy taking any steps to get it. (When I was a kid, I had to do a paper route, and collect discarded batteries to earn my bike!)
The fact is that imagining getting something is only the beginning. It works ONLY IF (big ONLY IF!) if that is followed by action to get it--and only if the outcome meets the well-formedness criteria for an outcome.
I'm sure that you don't advise your clients to simply imagine being slim, and that is all they have to do!!
Since you have already invited this person, I suggest that you pace the part about imagining the outcome (which is fine as a first step) and then add in the missing elements from the NLP well-formedness conditions. For more on this, read The Luck Factor--a great book that is the basis for actually teaching people the skills that "lucky" people have--very practical skills that do not have the misleading airy-fairy aura of the "law of attraction."
As you can probably tell, I'm on a bit of a rant here; I hate scams and deception, and I hate to see good people like you contributing to them in any way. And of course, perhaps your guest is just using "The Secret" as a hook, and then will add in all the missing pieces--but I doubt it. The rest is up to you.
"The most dangerous lie of all is the half-truth."
Steve Andreas, NLP author, trainer, developer,
(http://www.steveandreas.com) and art collector:
(http://www.charlespartridgeadams.com) 1221 Left Hand Canyon Dr. Boulder CO
80302 (303) 442-2902