My advice would be to pay her the 6 months of maternity leave and then to deal with whether there is any open position for her or not after she returns from her maternity leave.
Though she has not officially declared her intent to proceed on maternity leave, your post here confirms that you are aware that she is pregnant and hence the possibility for her to proceed on maternity leave. Who knows where all this information may be available and who all may be willing to testify for her, especially when you are bound to have disgruntled employees in the event of a lay off.
First and foremost, I recommend you using the right terminology in your post. Because of the reduction in the business, if you are required to reduce the staff strength, then it is called as "retrenchment" and not "lay off".
Now coming to the maternity leave of a woman employee. On the day when her maternity leave will start, will your business unit be functional or not? If not, then you need not give her maternity benefits. If there is no business activity in the unit in which she has been employed, then how come she will become eligible to draw her salary? Please note that "salary" is not disbursed just someone is at work. Employees get their salary even they are on leave also.
Going further, imagine a situation wherein a woman employee is on maternity and business unit is closed during her leave. After the day of the closure, should she get continue to get the maternity benefits? Certainly not!
I have given my opinions based on my general knowledge. However, I do not know any case law to quote.
You have written about the closure of the "business unit". When you say business unit, does it mean an independent business entity or just a section or department of a larger unit?