You have a right to maintain a dress code standard, even if you do not have a published policy per sé. You just need to be sure you are consistent in its enforcement and in no way arbitrary when you identify a violation of that policy. To that end, when someone is seen at work in an outfit unsuitable for the office you should seek out your HR resource, join them in a meeting with the individual inappropriately dressed behind closed doors, and discuss your concerns with the employee.
Be specific. Be polite and professional. In the worst case send them home to change.