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

Praise be to Allah.

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.