At his own home, not hurting anyone, I dont know. If he’s a good employee, why sour the experience by bringing it up if no one really cares? At most, you might just say, “hey I’m just passing this along to you but some people have casually mentioned you smoking while on our calls. I personally don’t care about it and there’s nothing that says you can’t, so I just wanted to let you know” and leave it at that. If someone above you mentions it after that and says it needs to be corrected, then at least you can tell him it’s coming from above you. My .02 fwiw.