I am one of the instructors in my club, and often come people to me asking for help,
like please adjust my plane, please test fly it, or if they are really new into this, they just want to see their own plane in the air,
and just have a great time looking..
But I always repeat: I will sure try, I will do my verry best, no matter what bad happens I am not to blame, and I will not pay a dime !
then they said: ok that is fair, lets give it a try.
Sometimes bad things just happens, and they always says: ok just bad luck and take it with a smile.
funny, they show up soon again to repeat the experiment, good luck that most of the cases it is with a happy ending.

Conclution: Always have specific clear rulez, so all know the consequences of their partisipation.

(sorry for bad spelling)