Just heard back from my friend (who retired from NASA last May) who finally got a reply from his contact at NASA. The second guy works in the Human Exploration and Operations directorate, and who asked my friend to "keep his name out of this" (I don't know why) about the exact type of film used to make the bag in which the photo was encased.
Hint, it is not "shrink wrap"
According to the contact at NASA, it was a polyethylene film that the Crew Systems Support division has used since the Gemini program.
The only thing the guy would tell my friend is that it is a "standard ASTM-D2103 polyethylene" film.
So, not having access to ASTM or their technical library, this is as far as I can get. I am going to try to see if I can get in touch with anyone at the library at HQ in DC to see if they can point me to the right technical document, although I've searched that database for several days now with no results. I don't know that this can be called a completely definitive answer, since it's second-hand hearsay without physical documentation, but at least I know what I am looking for now. It's easier to prove something when you know exactly what it is that you're looking for.
I must say though, I am surprised that everyone is so ready to accept a You Tube video as "evidence"