I don't mind if someone is stern with my 2 (DS especially), in fact I think it reinforces the message if it's coming from someone else. DS is a cyclone and easily loses his temper, I really value it if someone else pulls him up if I'm across the room. (this doesn't happen all the time, he is quite the angel at kinder ).

ETA - having said that, I wouldn't want someone being horrible to him and I would not be happy if they were rude. Stern ok, if appropriate.