And finally, lets face it, pilots like to talk about the job and about themselves and how they got into flying.
If I take a friend on a jumpseat (always cleared beforehand with the Captain) I always tell them to ask the Captain any questions they may have, like technical stuff or whatever. Don't get too deep into the technicalities though.
So even if you think you know what the system is and how it works, it is worth asking the pilots.
I prefer someone to ask stuff rather than sit there keeping quiet the whole flight.
Take an interest but realise that some of the stuff is really going to be not relevant. They know you are a PPLer so know you know some stuff already.
I mean, don't start querying the approach briefing, perhaps listen in and then on the way back, if you want to know more, ask then.
It might be worth asking your mate to do you a photocopy of the relevant approach/airport plates, it makes it more interesting if you can see the approach they are doing.
Some pilots will chat briefly to you in the climb and descent, if you reply just keep it short.
And never, ever, say "have you ever crashed" or "have you seen any UFOs" and to the FO never say "so when do you get to be a pilot then?".
Thanks.
Oh yes, and don't get in the way of the cabin crew either during the flight or on the turnaround or you will be extremely unpopular!