By submitting evidence about the procedure rather than the procedure itself: revision histories, training records, project lists, an explanation of what it does and why it is company-specific, and a description of the technician's role in creating it.
Choosing what to disclose
Ask the shop manager to produce a one-page description of the procedure in plain language: what problem it solves, when it was introduced, how it differs from industry practice, and which products depend on it. That page is the spine of the file. Around it, place the document control log showing revision dates and authors, the sign-in sheets from internal training the technician delivered, and a list of jobs where the procedure was applied, with dates.
None of those exposes the tolerances or the fixture drawings. If a reviewer asks for more, the company can decide then whether to provide excerpts under a confidentiality request. Payroll records and the employment contract prove the year abroad; the corporate register proves the relationship.
Redacting so heavily that the reviewer cannot understand what the knowledge is defeats the exercise, so the description must be specific about function even where it is silent about specifications.