In addition to just stating it was okay for that use, it should also have had some standard or approval numbers, such as ASTM, ANSI, ICC, "something" to indicate it had "third party" testing and approval by some authority.
If all it had was writing stating it was okay to use for a T&P discharge pipe, I would write it up for the plumber to make appropriate repairs or to provide documentation that this is suitable for this use.
Many times, if the plumber is not familiar with the material, instead of spending time to find the documentation, they will just replace it. If they have already had to supply this documentation to the AHJ, they will just whip out that documentation and say something to the effect of 'Jeez, I've already given this stuff to the plumbing inspector, how many more people do I have to show it to?' (Answer: Anyone who asks for it.
That's the perils of using non-standard material and trying to save 50 cents.)