HR could send out a general messege to the entire staff, that says:
Every employee is expected to come to work clean and in business ( or what ever attire) each day, Please keep this in mind when preparing for work each morning.
Thank You,
HR Department
This way, it does not single out any one person, and you are not involved in telling her ( nor should you discuss it with other employees) then, if it continues in the next week or so, then the HR manager can call in that employee and have a closed door private talk with the person without anyones knowledge.