Praise be to Allah.
The owner of the restaurant is responsible, according to sharee’ah, for what happens in his restaurant of sins that he could stop or prevent from happening in the first place. The prohibition of alcohol is proven in the Qur'aan and Sunnah and according to scholarly consensus. The Prophet (peace and blessings of Allaah be upon him) cursed ten people with regard to alcohol, not just the one who drinks it.
It was narrated that Anas ibn Maalik (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) cursed ten types of people with regard to alcohol: the one who presses (the grapes, etc), the one for whom that is done, the one who drinks it, the one who carries it, the one to whom it is carried, the one who offers it, the one who sells it, the one who consumes its price, the one who buys it and the one for whom it is bought.
Narrated by al-Tirmidhi (1259) and Ibn Majaah (3381); classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
This proves that the sin includes everyone who contributed towards its presence, so that it may be drunk at the end.
The one who sits in a place where Allaah is denied and where evil actions and sins are committed is a participant with them in that sin, if he is able to change the evil but does not do so, or he is able to leave that gathering but does not do so. Allaah says (interpretation of the meaning):
“And it has already been revealed to you in the Book (this Qur’aan) that when you hear the Verses of Allaah being denied and mocked at, then sit not with them, until they engage in a talk other than that; (but if you stayed with them) certainly in that case you would be like them. Surely, Allaah will collect the hypocrites and disbelievers all together in Hell”
Ibn Katheer (may Allaah have mercy on him) said:
i.e., if you commit sin after it has reached you, and you agree to sit with them in a place in which the verses of Allaah are being denied and mocked and belittled, and you approve of them doing that, then you have a share with them in what they are doing. Hence Allaah says “certainly in that case you would be like them” i.e., in sin, as it says in the hadeeth: “Whoever believes in Allaah and the Last Day, let him not sit at a table where alcohol is being passed around.” End quote.
Tafseer Ibn Katheer (2/435).
Allaah enjoins the Muslim to denounce evil when he sees it or knows of it taking place. So how can this coincide with allowing those customers to drink alcohol in your restaurant?
It was narrated that Abu Sa’eed al-Khudri (may Allaah be pleased with 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.”
Narrated by Muslim (49).
Another of the attributes of the slaves of the Most Merciful is that which Allaah says of them in the verse (interpretation of the meaning):
“And those who do not bear witness to falsehood, and if they pass by some evil play or evil talk, they pass by it with dignity”
According to some scholars – such as Imam al-Zuhri – what is meant by falsehood is drinking alcohol.
See: Tafseer Ibn Katheer (6/130).
Based on that:
You should put a stop to this by not allowing anyone to drink alcohol in your restaurant, otherwise you will be taking upon yourself the sin of those drinkers. Do not hesitate about this matter, and you will see the effect of that in your religious commitment and in your provision (rizq), if you seek reward from Allaah in this matter.
And Allaah knows best.