"Professor Portal" achievement in Portal 2

Question

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... :(

Answer 1

No, it's not ok, I tried several times. And the time I did get the achievement was when I added the guy to friends list before I started the game with him.

Answer 2

snip

I previously stated that it was not necessary to be friends with your co-op partner. This is however false information.

EDIT: You get the achievement at the end of the track when the confetti falls from the ceiling. If your co-op buddy is in your friend-list by that time you will get the achievement. So it's all good to add your co-op partner as a friend during the course.