Whatever the rights and wrongs of letting a passenger have a go at the controls, check
this accident report.
Nothing in there to say if the passenger WAS having a go or not, but the most likely scenario the AAIB can come up with is a control input which led P1 (a qualified pilot, like all of us) to react in such a manner as to lead to the failure of a wing.
Now - still want to let your rellies have a go ?