You should not return the person to her job without a return to work certificate or letter from the employee’s primary physician indicating that the employee may return to work and any work restrictions that the doctor advises.
This should also include the length of time these restrictions apply. If restrictions are involved, the employee’s visits to see if the restrictions can be removed should be at least as frequently as monthly. Insist on that. A good way to do this is to supply the primary physician with a job description that includes the physical requirements of the job so that the physician has that information on which to base his/her advice regarding restrictions.
If no restrictions are recommended by the primary physician, you can insist on the employee returning to work on a full-time basis--but be prepared for resistance on her part.
If restrictions are recommended by the primary physician, you must determine if you can reasonably accommodate the restrictions. If you have 15 or more employees, you must abide by the Americans with Disabilities Act and/or the Americans with Disabilities Act Amendment Act in these regards.
If your assessment indicates that you cannot accommodate the restrictions advised by the primary physician, you can terminate the employee. But be aware that the best course of action is to consult with an attorney who specializes in employment law in your state before taking that step.
There will be exposure to a claim of age discrimination and/or discrimination under the ADA/ADAAA if the employee is so inclined to take umbrage at being fired because she was hurt on the job.
A more prudent course of action might be to find a way to accommodate this individual for the time being and see if she is able to do her full job within a reasonable period of time.
You might hire a temporary part-time employee to pick up the slack, for example, instead of continuing to overburden other regular employees indefinitely.