A dated copy of the official page and FAQ from the day of each payment, the payment confirmations, the vetting correspondence and a written note of which immigrant category is being used. If the terms change later, that record shows what she relied on.
Document the terms because they live on a web page
Unlike a regulation, which is published in the Federal Register with an effective date, the Gold Card framework is set out on an official website and FAQ that can be revised without notice. An applicant who pays a processing fee in one month and is asked for a gift in another should be able to show exactly what the page said at each point. The simplest discipline is to save a dated, complete copy of the page and the FAQ on the day of every decision or payment, print the payment confirmations, and keep the vetting notices and any correspondence with the program in one folder.
Add a short memorandum recording the category that will carry the case, the visa-bulletin position on that date, and the per-person amounts calculated for the household. This is not suspicion of the program; it is the ordinary care anyone would apply to a seven-figure non-refundable payment made on terms published outside a statute.