Short and simple answer: just say "NO".
Long and a bit more complicated answer:
Once had similar request from a friend. This guy is a frequent LH flyer (for business), but scared of flying. Wanted to see how he would cope in a SEP. Now, he's a tall, strong guy, not someone you want to grab the controls and freeze. Solution came in the form of his girlfriend, who, by dint of fate, had been flying in little planes since childhood and loved it. So: girlfriend in the front right seat, the tall, scared one in the back. The drill was he would tell me immediately if anything felt wrong and I would head straight back to the airport, no questions asked. Guess what: the flight turned into a two-hour sightseeing trip which all three of us loved.
My guess (pure speculation, I have no proof for this) is, that many people who say they are scared of
flying actually suffer from claustrophobia and/or some form of agoraphobia and hence feel very, very uncomfortable crammed into a metal tube with a lot of strangers. Add to that the feeling of being at someone's (the crew) mercy, and you got a recipe for 'freaking out' (especially on a holiday charter
)
Being in a small a/c with big windows and a pilot they can talk to is different.