As other posters put it - it can vary.
Our dojo starts Hakama at 3rd kyu if you want - but at that point your pretty good already.
Even though its not that easy to spot initially, a black belt still has his black belt on with a Hakama, and a kyu rank wears a white belt typically. (See on the sides of the hakama.)
Probably, when in Rome, he should do as the Romans.
i.e. - wouldnt have been that difficult for him to see your dojo standards and just leave his hakama in his sport bag.