You can definitely fit a friend on the 787 F seat for a quiet drink or two and I did so recently. They have to be happy to perch on the foot locker. If you time it right the crew usually oblige happily and offer a drink - but please don’t put them in the awkward position of asking your friend to leave the cabin - and keep it to the two drinks or so mark. It’s a nice gesture, not to be taken advantage of.
It goes without saying not to try this on an overnight flight when other F passengers nearby are sleeping.