Evidence that the funds are lawfully hers and that the path they travelled is visible: corporate and personal tax filings, the sale or distribution documents behind the money, and dated statements for every account the money passes through.
Where the money came from is a question in any route
Whatever route an owner eventually takes, the financial record is the part that takes months to assemble and can be started immediately. For a business owner that record has a predictable shape. The ownership history of the company, shown by registers and any purchase or subscription agreements.
Financial statements and corporate tax filings for several years, establishing that value accumulated through ordinary trading. Personal tax filings covering the same years. Then the transaction that released the money — a sale agreement with its closing statement, or a documented distribution properly recorded in the company's accounts.
Finally, dated bank statements for each account the funds pass through, including any currency conversion. Owners frequently find that one link is missing, most often an internal transfer nobody documented, and locating it early is far easier than explaining a gap later.