How about:
"Well, keep doing what you're doing. It's clearly working out wonderfully for you!" (Insert double eye roll.)
Ha! Well, there are reasons why I'm not running a charm school!
In real life, I think foodaddict's offer of "would you like to read a book" is the nicest and gentlest! It seems inappropriate to challenge people or try to back them into a corner, or try to convince them that they don't know what the hell they're talking about. That just shuts them down.
If I picture someone questioning me on how I'm leading some area of my life (say, someone questioning me on my decision to abstain from health insurance) -- I would be extremely put off by someone asking me, "If you knew the stats on bankruptcy and the uninsured, you would change your mind about what you're doing". Or i would be put off by someone saying, "Have you actually priced out health insurance? You might realize that it's not as expensive as you think".
Anything that suggests that you know what's best for me, and I'm too stupid to lead my own life -- is a stinker. By contrast, anything that suggests that there may be more options than I am aware of, might feel nice. Or suggesting that when I am ready to try a different option, you may have some ideas for me -- that would feel fine to me too.
I mean, until I give some indication that I'm unhappy with my decision, then I'm not positive that there is a teachable moment there.