I believe that legally they can't force her to use sick leave since she has met the 40 hours minimum required as a salaried employee. Is this correct?
Unfortunately, as far as the Department of Labor (DoL) is concerned, the employer is correct here, provided that this is company policy.
First, there is no "40 hours minimum required", the DoL simply says that a salaried (exempt) employee must be paid the full salary for any week in which the employee performs any work, regardless of the number of days or hours worked. This is then limited by the "allowable deductions:
Circumstances in Which the Employer May Make Deductions from Pay
Deductions from pay are permissible when an exempt employee: is absent from work for one or more full days for personal reasons other than sickness or disability; for absences of one or more full days due to sickness or disability if the deduction is made in accordance with a bona fide plan, policy or practice of providing compensation for salary lost due to illness; to offset amounts employees receive as jury or witness fees, or for military pay; for penalties imposed in good faith for infractions of safety rules of major significance; or for unpaid disciplinary suspensions of one or more full days imposed in good faith for workplace conduct rule infractions. Also, an employer is not required to pay the full salary in the initial or terminal week of employment, or for weeks in which an exempt employee takes unpaid leave under the Family and Medical Leave Act.
(Source: dol.gov) Emphasis Mine
This means that as long as the employer has a policy or practice requiring the employee to use Paid Time Off (PTO) for sickness (sick-days), then the employer is allowed to make deductions from the employee's salary for those days.
So in short, yes, the employer can require that the employee use PTO to cover sick days, regardless of the actual number of hours worked in that week, month, year, etc. There is currently no federal requirement for employers to provide paid sick leave, although some states like California may have local laws.