I think its a good opportunity to discuss with your son what the other child was doing and to, in a light and good humoured way, reiterate that you do things differently in your family. I have had a similar problem...only it is ds cousin! Haha. For example, ds might say, "x threw sand and said y was poopy!". So I would say, "that's silly isn't it! We don't call people poopy or throw sad, do we?" seriously, the conversations you have with a four year old! My son is pretty matter of fact about things and doesn't really get the immature stuff, so I am lucky this is all I have had to do to keep him behaving.