Pieper used Cockerill steel, as did most of the U.S. double gun makers.
Yes, certainly 65mm chambers.

Your friend might review this
https://www.shotgunworld.com/threads/is-my-gun-safe-what-load-was-my-gun-designed-to-shoot.366087/
The gun shouldn't be used, with any load, until it has been evaluated by a double gun specialist with the interest, equipment (bore scope and wall thickness gauge) and expertise to properly do so.
"Should be OK with light loads" is not an expert assessment.

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