He brings them juice without the owner of the restaurant knowing; what should they do?

It is not permissible for a worker to take anything from the
restaurant for himself or his friends except with the permission of the
owner. That is because the Prophet (blessings and peace of Allah be upon
him) said: “It is not permissible [to take] the property of a Muslim except
with his consent.”

Narrated by Ahmad, 20172; classed as saheeh by al-Albaani in
al-Irwa’, 5/279. 

Imam Ahmad also narrated (23094) from Abu Humayd as-Saa‘idi
(may Allah be pleased with him) that the Messenger of Allah (blessings and
peace of Allah be upon him) said: “It is not permissible for a man to take
his brother’s property unlawfully, because Allah has forbidden the property
of one Muslim to another.”

Classed as saheeh by al-Albaani in al-Irwa’, 5/280 

And it is well known that the owner of the restaurant would
not give permission for such a thing. 

Based on that, it is not permissible for you to drink this
juice, and it is not sufficient for you to give its price in charity. If the
owner of stolen property is known, it is not permissible to benefit from it;
rather it must be returned to its owner. 

The scholars of the Standing Committee for Issuing Fatwas
were asked: 

My father works in a restaurant and the owner of this
restaurant is stingy, hence my father – along with other workers in the
restaurant – take some of the food without the owner’s knowledge. My father
brings 3 kg of meat every week without the owner’s knowledge. I said to him:
Why do you do that? He said: Because the owner of the restaurant is stingy
and he does not show us any kindness. I am a student who is still studying.
Is eating this food haraam? Please note that this food usually stays in our
house for approximately 4 days, and we do not eat anything else. 

They replied: 

It is not permissible for you to eat this food that your
father takes secretly from the restaurant without the owner’s knowledge,
even if the owner of the restaurant is stingy, because the worker is not
entitled to anything except his wages and other things that were stipulated
in his contract of employment. Based on this, it is not permissible for you
to eat anything that your father has stolen from the restaurant, because the
Prophet (blessings and peace of Allah be upon him) said: “The whole of a
Muslim is sacred to another Muslim, his blood, his wealth and his honour.”
Narrated by Muslim in his Saheeh, 2564. end quote. 

Fataawa al-Lajnah ad-Daa’imah,
22/336-337 

They also said in response to a similar question: 

It is not permissible for you to consume any of the trade
goods except with the permission of their owner, because it is proven that
the Prophet (blessings and peace of Allah be upon him) said: “It is not
permissible [to take] the property of a Muslim except with his consent.” End
quote. 

Fataawa al-Lajnah ad-Daa’imah,
14/29 

So you have to refrain from drinking this juice and advise
your friend to repent to Allah, may He be exalted, and stop doing this
haraam action. 

With regard to what he has previously taken from the
restaurant, he should either ask the owner of the restaurant to forgive him
or he should work out the value of these things and give that money to the
owner of the restaurant. It is not essential for him to tell him what he has
done; rather it is sufficient for him to make sure that it reaches the owner
by any means. 

For more information please see the answer to question no.
40157 

And Allah knows best.