Why not? does it bother you? if it does why are you hanging out with them. It's not really anybodies business to tell someone else what they can or cannot do with their bodies, right! But if you are concerned for your friends tell them the consequences of smoking and what it will do to them. Then let them do as they please, as a friend you have done your duty. As a Sikh we are taught that the body is our Temple, by the very rules we follow on what we can eat drink and/or take into our bodies this is shown to be one of the most important aspects of our reliogion. God gave you this body as a gift dont ruin it with pollutants. Therefore if you have friends who smoke ... fine... just dont get involved yourself. Thank You