What your friend is doing is not one of dummy's rights as per the Laws. He is not trying to stop an irregularity: he is merely contributing to the general noise level in an unnecessary and provoking way.
There is a common failing amongst dummies that they try to get more involved in the game than hte Laws allow, for example when dummy is all winners but declarer has not noticed they say "You have the rest". It is an unfortunate failing.
In France they call dummy "le mort" which means the dead one and that is a good description of the best dummy. To quote David Burn, a top English player, dummy should be at the bar buying the drinks.
So someone should explain to your friend that he has no right to do this and should desist.