Is it law that a company, despite its size, provide maternity leave pay or is it a benefit decided on by the company, like the number of paid vacations/year? I'd imagine it's not a federal law, or even a state law anywhere, because I just can't imagine a place like McDonalds providing maternity leave pay for the cashier, for example. If it's not law, it can be explained to any women looking for employment with you that you don't provide maternity leave pay if they should happen to be with child. Or perhaps the position you have open doesn't receive that benefit? Or you can say you don't have that yet if you want to sound proactive
Maybe they can come back to the job when they're ready? That way they have job security (such as it is) and you can funnel the money for her into a temp or something.