Employment Law Questions? Ask an Employment Lawyer.
Yes, it can be legal, depending on the circumstances.
If she is unable to work right now at her regular job due to the injury, there is no law requiring the employer to create a light duty position for her. They can require that she be able to do her regular job and if she is unable, then the employer can stop her from working.
That missed time would then be missed based on a medical condition and an employer, when aware of a qualifying medical condition, can inform an employee that they will be counting the time as FMLA time. The FMLA law specifically gives the employer the right to require the use of FMLA for qualifying time off.