Agree with Mr. Dinesh, maternity leave is an employee's right that cannot be linked with her joining back to the employer.
Steps to Address Non-Return After Maternity Leave
Now, with regards to your query, please send her a letter asking her to rejoin as her maternity leave is exhausted. If you don't receive any reply, then send her a second letter stating that if she does not rejoin within the stipulated time, it would be assumed that she is not interested in the job, and the company will terminate her employment. After that, if she still does not reply, you can send her a termination notice for abandonment and deduct the notice pay from her FnF.