As I stated before, the decision as to who IS P1, is a critical one, and all of us PPLs should make it an item on our pre flight check list.
I for one have experienced in a mild way the complications of this when many moons ago i flew (P1) with a friend (P2) who wanted to check out his new handheld GPS and took on the "responsibility" for the navigation, getting us into Luton airpsace without clearance..... I was P1 and got (rightly) the flack from the controller over the phone (no prosecution).
I knew i was P1 and didn t check P2's actions... My fault.