Your friend probably doesn't sweat very much, if at all. If that is the case then they wouldn't need to use a deodorant. Other people sweat more and using a deodorant helps them to stay dry. There are also some people who may sweat but it doesn't smell. If we wash ourselves every day, then for some of us we would not need any deodorant at all. I always put deodorant on and then talcum powder.