In coaching we teach our clients the two breath method. Take two breaths before responding, this will give you an opportunity to calm down a bit and not just react. This is an extremely good idea for ALL to learn in order to give your lips a chance to stop stupid comments from coming out of your mouth and your brain a chance to sort them from smart ones.
People say the stupidest thing at shiva homes. They will tell men who have lost their wives to call them after Shiva because they have a good shiduch for them. They will ask women who have lost their husband if they are putting the house of for sale because they might have a buyer and even more stupid things if you can believe it. People should learn to either keep their big mouth shut or just follow the lead of the aveil. Or they should practice or prepare of few decent things to say before they go. As others have said, if you don’t have anything nice to say just smile and say nothing. Your presence is enough.