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

(a)

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. 

(b)

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”

[al-Nisa’ 4:140]. 

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). 

(c)

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). 

(d)

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”

[al-Furqaan 25:72]. 

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.