I achieved almost all of Portal 2 achievements, except "Still Alive" and "Professor Portal". The first one is not a big problem but second one is, since my steam friends who have Portal 2 have already played co-op.
One way to achieve this is to play with random player and become their friend, however, I'm wonder if becoming friends after starting the calibration course is still accepted.
Is it OK (for earning "Professor Portal" achievement) to become friends with another person after starting the calibration course?
PS : sorry, I can't decide the answer until saturday because I can't confirm until then... :(