Just open sourced it: github.com/mr-guac/redactpdf

For your friend's air gapped environment, the file works offline after the libraries cache on first load, but it does pull PDF.js and pdf-lib from CDN so a one-time internet connection is needed.

To run it fully offline you'd need to download those two libraries separately, transfer them to the air gapped machine, and swap the CDN links in the HTML to point to the local files instead.