Process manuals and recipe files with her name in the revision history, batch and trial records she signed, the training register listing operators she qualified, and a written comparison of what a new hire would need to learn.
Authorship and training records are the proof
The strongest L-1B evidence is contemporaneous and internal. A manual whose revision table shows the technologist as author across several versions proves both that the process is the company's own and that she developed it. Trial records from the commissioning of the current line, with her signature on the parameter sheets, tie the knowledge to actual production.
A training register showing which line operators she qualified, and on what, demonstrates the company relies on her to transmit the knowledge, which is exactly what the U.S. assignment proposes. Add an organizational chart placing her in the quality and product-development function, and a short statement from the plant manager explaining how long an externally hired technologist would take to reach her level.
Where customer recipes are confidential, redact quantities but keep structure and dates visible; the adjudicator needs to see specificity, not formulas.