I want to know if our contractor used the proper materials behind our "direct-to-stud 3-piece tub surround (it has a seamless look, but it's 3 panels for the 3 walls plus a bathtub).

Behind the tub surround, the contractor installed regular QuietRock soundproofing (he didn't use QuietRock ES Mold Resistant). Then he installed XP Gypsum (purple) Board. Then the tub surround walls. He installed purple board for the walls above the tub surround over the flange of the tub surround.

Is this setup fine / up to code, or should he not have installed purple board behind the tub surround panels?

The Gypsum Board manufacturer's web site states that "XP Gypsum Board
should not be used as a backer board directly behind tile and wall panels in tub and shower areas." I also read on a web site: "When installing a monolithic tub and shower enclosure (e.g., fiberglass with no seams), know that cement board or an equivalent is not required, but all paper-faced backerboard must meet ASTM mold-resistant standards." Well, I don't have a monolithic tub enclosure, so according to this site, you're not supposed to use paper-faced backerboard behind a non-seamless tub/shower enclosure. However, my enclosure is a direct-to-stud type of installation, so I don't know if this also applies to my enclosure.

Since the contractor had to soundproof the walls, he installed purple board over the QuietRock. I'm totally confused as to whether or not he should have installed the purple board behind the tub enclosure or whether he should have installed cement board along with a waterproof coating.

Thank you so much for any info and help.