A dated copy of the official program page and FAQ as read at the time, the official payment instructions, receipts for the processing fee and the gift, all correspondence from the program, and the applicant's TN admission records for the same period.
Preserve the official instructions and every receipt
Because the program's terms are published by the government and may change, the applicant should print or save the official page and FAQ with the date each time he relies on them, so that a later reviewer can see what the rules were when he acted. Keep the official payment instructions and use only those, never account details forwarded by a consultant or received by email without verification. Retain the receipt for the US$15,000 processing fee, any vetting correspondence, the instruction to make the gift and the confirmation of the US$1 million transfer, and every notice received afterwards about the EB-1 or EB-2 step.
Alongside those, keep the architect's TN documents for the same period: employer letters, admission records and any I-129 approval, since the two files may be reviewed together at a later admission or at the residence stage. Hypothetical example: if the program later asks how the gift was funded, the architect's own bank statements showing the money leaving his account will be needed, so keep those with the receipts rather than in a separate tax file.