What you would do is create a series of payment requests for her, each with a different date. You'd have to warn the guest that these requests are going to generate emails, but she can ignore those until it's the right time to pay them (and reminders will be sent automatically at the right time). Then delete the payments that were automatically created as part of the reservation to bring the total in line with what you require. And make sure your last payment is the same number of days from arrival as it would have been otherwise.
Yes, it's a hassle, but I've done it for guests who are trying to manage their cash flow. Makes for a happy guest (usually).