If you are not required to be there I don't see how you can log it. It would be like if a 747 captain let you touch the controlls and you decided you wanted to put .5 747 time in your logbook. And always remeber if you have to explain any time during an interview. I woudn't think trying to tell your future employer that all your multi time came from a buddy who let you manipulate the controls. There is a lot more to being the PIC the just flying along in cruise.