Small Home Game talking during a hand

[tennisjump]

I host a small nl 25 game with 5 of my friends that's very casual because most of them don't really know anything about poker. Obviously it's wrong if someone folds out of turn, but as long as it's not a regular occurence, I don't really care. One of my friends though has a habit of talking to another player during a hand and if its bet to him and asking if they're going to fold or call. Now I've told him he can't do it, but he says that they don't have to answer him. What would be the best way to explain that its collusion?