It is obligatory to denounce the smoker and prevent him from smoking in the café, because of the report narrated by Muslim (49) according to which the Prophet (blessings and peace of Allah be upon him) said: “Whoever among you sees an evil action, let him change it with his hand (by taking action); if he cannot, then with his tongue (by speaking out); and if he cannot, then with his heart (by hating it and feeling it is wrong), and that is the weakest of faith.”
The owner of the café should not only put up a sign saying that smoking is not allowed; rather he should strive to enforce this message and not allow anyone to smoke in the café, because smoking is haram and also harms others, and taking the matter lightly is encouraging others, in which case there is no point in putting up a sign.
If anyone sits in the café and looks at the women passing by in the street, his sin is upon him only, and the owner of the café is not responsible for that.
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