A dated copy of the official program page and FAQ as they read when he paid, the payment confirmations for the gift and the processing fee, the herd and quota sale records, and any written answers from the program or an adviser.
Keep the terms as they read and the money as it moved
Two sets of records matter. The first is the program's own terms, because they are administered through an official page that can change: print and date the page, the FAQ and the application instructions, and note the amounts, the stages and the family terms as they read that day. Keep the confirmations for the processing fee and the gift with the dates and the account details used.
The second set is the money's origin: the sale contracts for the herd, the marketing board's quota-transfer confirmation and payment record, and the bank statements showing the proceeds arriving and the gift leaving. The program does not require an EB-5-style source-of-funds trace, but a bank, a tax authority or a later immigration process may ask, and the records are cheap to keep now and hard to reconstruct later. Add certified English translations for any French document that may be requested.