Answer:- The interior designers task is to prepare the outlook and fittings according to the usage of a place. So if a room is used as a dining room, he designs it in a manner that suits the needs of a dining room and if it is used as a office space, then he designs it so as to suit an office and in a similar way he designs the bar and restaurant in a manner than suits the restaurant and the bar. So he designs and applies the necessary fittings for the bar and the restaurant which includes the fixtures and lightings and the tables and stools and so on for the bar area as well. In other words it is his work that will make the area ready to be used as a bar to serve alcohol.
And as we know that Allah (swt) said: “…but do not help one another in sin and transgression…” [al-Maa’idah 5:2]
If his work was to design a restaurant it would be a different case which would be studied in a different manner depending on what is served in the restaurant but since his work also involves desiging the bar which would only serve alcohol, this therefore would constitute helping others in sin and transgression and therefore is not allowed.