Since there is no contract of employment, she is typically an employee at will, unless the employee handbook indicates otherwise.
As an employee at will, the law permits you to implement and enforce policies with regard to when the employee must be at work and when they may take sick days or vacation days.
So as long as you provide notice as to what you expect and do not discriminate on the basis of race, gender, age, marital status, or other protected classifications, you are normally free to take whatever action you wish with regard to violations of your policies, as far as a written or verbal warning or even termination.
If the employee works a partial day, you should typically not dock her though since you would risk losing her exempt status. Instead, if she does not regularly work overtime, you may want to begin paying her on an hourly basis.
You can dictate when she must use sick or vacation days normally, but if she utilizes all days to which she is entitled, you should not typically dock her pay for partial days as long as she is a salaried exempt employee.
Good luck and take care.
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