Work dried up during maternity leave: Can we terminate?

Q: One of our employees is on maternity leave, normally working part time for our billing department. When she left, we assigned someone else to her job, and now the status in our office has changed and there is lack of work for her because we lost a large customer for whom she provided billing services. The question is, do I have to bring her back? If I do not, can I let her go? – Doreen, New Jersey