I agree that the client was rude, to take a cupcake before being offered.
I think it would have been polite for associate 1 to decline the cake as it was his client who had taken 'his' share, but that assumes that in the time between the plate being offered and his taking a cake, he had noted that there were 5 other people (plus his client) and only 5 more cakes.
I also agree absolutely that if there is a hierarchy then the more senior person ought to ensure that their juniors have a share before they dig in.
I'm mildly curious about the set up of the offices, though - was client just arriving or leaving? Otherwise it seems odd that the new tenants could walk in on a private meeting with a client (even bearing gifts!)