Is one allowed to borrow money from some one works in a restaurant which
sells haram things? If he borrowed this money already what is the hukm?

Yes, it is permissable for a Muslim to borrow from someone in such a situation. The daleel
(evidence) for this is that the Prophet
(peace be upon him) borrowed from the Jews
and they were dealing in riba. Furthermore, in most cases, such a person’s income is not
totally haram, because it is a mix of earnings gained by selling both halal and haram items.
However, you should advise him and clarify to him the Islamic ruling on what he is doing.