I find that the best way to deal with someone who is rude enough to do that (including a boss!)is this: Pick up at least 2 Serviettes while they are blowing their trumpet. When they have finished, silently offer them 1 Serviette and tap your eyebrow as if they have missed their nose.
They will take the tissue from you & wipe their own eyebrows and face with it (in front of everyone) and nod their appreciation to you. Count to 20 & then catch their eyes again, but now tap your chin, offer the second tissue and they will again accept it but this time feel extremely conscious, they WILL wipe their chin and look to you for approval. At this point, very slowly shake your head (No) while looking them in the eyes directly. Then turn away... You will see that they wipe their whole face again and do so in an embarrassed state. They seldom blow their nose during diner again... hahaha. It Works!