Log it as you please - unless you try to log it as PiC which, as someone being 'checked out' by someone else you cannot reasonably have been.
I would log it as P1u/s - that's what you were. But it's only PiC time which matters for 'hours building', so it doesn't affect the price of fish at the end of the day! It will, of course, count towards your SEP experinece total for revalidation, but not if you were just a passenger with someone not qualified to instruct in the other seat.