What is the ruling regarding my relative working as an accountant in a women's salon that performs acts contrary to Islamic law, such as plucking eyebrows, knowing that she advised the owner of the salon to no avail?

It is not permissible for a female relative to work in matters related to the forbidden within a women's salon that combines the permissible and the forbidden. She must leave the job if the employer does not accept her working only in the permissible aspects, unless she is compelled to stay due to an urgent necessity. In such a case, she may remain until she finds another job that alleviates this necessity.