If he allows people who bring alcohol with them to drink it in his restaurant, is he sinning?

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Check below answers in case you are looking for other related questions:
Eating with people who are drinking wine at the same table
How should she deal with her husband who drinks alcohol?

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Related Answers:
Woe to the drinker of wine
Concealing people’s sins is a matter that is subject to further discussion
How to denounce evil in one’s heart
Eating with people who are drinking wine at the same table
How should she deal with her husband who drinks alcohol?

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