Bit of a silly statement. The boss appeared to be looking after the welfare of his employees by not letting them work in dirty environment. How does that put him in the wrong?You say it was 'such a hole' that the boss said no-one was to work there until it was cleaned up. He is just as bad except he did not post a picture. ok your friend should not have posted the picture but to say 'would you have a coffee here' does not seem like gross misconduct to me. Surely she would require a verbal warning then a written warning before being dismissed.
As for dismissals, you don't necessarily need a process of verbal then written warnings before being shown the door.