Is money earned from working in a café and restaurant where forbidden mixing is prevalent lawful or unlawful?

Working in a restaurant and coffee shop that does not sell forbidden items is permissible in principle. However, if there is excessive unlawful mingling (ikhtilat), then working there becomes forbidden, though the wages remain lawful. It is prohibited to work there unless the mingling is rare, or if you are compelled to work, in which case you must denounce the evil and search for another job free from temptation. Forsaking a job for the sake of Allah will result in Him providing you with something better.