Don't tell them and if they do push the point always call it off saying the wx isn't good enough because you don't want to scare them again. And when they express disappointment suggest they go up with an instructor instead because they are used to deal with this sort of crap and they would get more out of it.
As an ex instructor I really would advise you not to get involved with this sort of problem. Most of us have a war story about having someone with a similar tale going daft on us. 99% of the time we can make a difference and all is well but the 1% are a pain in the arse. And I don't think you are the type to belt them round the head with the fire extinguisher.