As an Instructor I took several people with a fear of flying on trial flights.
Luckly everyone had a double slot booked.
I used to do everything with them went through the wx, did a full pre flight check the works.
They were all nervous granted but most of them I managed to con into doing the takeoff and most of the flying; and one or two gave it a good shot at the landing. And one did a bloody greaser with no input from me.
Even the ones who said they didn't want to fly at all had a great time and all said that they where alot happier after everything was explained to them.
From my limited experence most of the fear came from the fear of the unknown and having to put complete trust in people that they never get to see. Once they speak to the pilots and get to know them they are fine. Once they have met one pilot and seen them work some of the fear is still there but it is controllable.
So go for a trial lesson but let them know the reason why you are taking it.