Deputy manager Joanne Huang told waitress Jahnayde Henry she looked 'tired and unpresentable' at the beginning of one shift and that she should wear makeup in future.
After working five shifts at Tattu, Henry resigned saying she felt bullied and her confidence had been undermined.
She then unsuccessfully sued the restaurant for race discrimination and harassment.
However, her claim of sex harassment regarding the makeup comment was upheld.
Michelle Last, employment partner at Keystone Law, said the deputy manager's comment was sexual harassment because it was specific to Henry being a woman.
Speaking to HR magazine, she said: “The law is that a firm line manager that treats all employees similarly will not normally be found to have harassed an employee who takes exception to a strong management style.
“However, the claim for harassment for suggesting Ms Henry should wear some makeup, was found by the tribunal to be a sex-specific comment. Although it is not unknown for men to wear makeup, it is not a comment that would ordinarily be made to a man.”
Last said this case highlights how dress codes could be discriminatory.
She said: “This case emphasises how difficult it can be for employers to impose a dress code on employees, particularly for dress codes that are intended to make an employee appear more attractive.
“For example, dress codes that require a woman to wear makeup, a skirt, heels or similar, may be discriminatory on grounds of sex or amount to harassment.”
She added: “Employers that base their business model on their employees appearing attractive, face substantial risks of successful claims.
“To avoid the risk of claims, employers should have a well-written dress code policy, with no requirement for any dress code that may amount to discrimination or harassment.”
Henry’s compensation will be decided at a later date.
Tattu has been contacted for comment.