What is the ruling of the Sharia on a wife working for a non-Muslim, unmarried man, sometimes drinking tea with him, and her refusal to leave this job despite her husband's displeasure?

The wife's work is permissible with conditions, but your wife's work as described is forbidden for several reasons: due to her mixing with a non-mahram man (and a disbeliever), her drinking tea with him (which suggests seclusion or emotional closeness), your disapproval of her work, and her leaving the house without your permission. This is sufficient to deem it forbidden.