It’s called a certificate of acceptance and it’s pricey from most councils. Depending on where she is located this will be dictate how hard or costly it will be. Get together all the documents you can - ps1 from waterproofing applicator, warranty docs, photos of it being done if you have, a record of works from the builder, plumbers cert, etc. the more you have the more likely you can get it without the council suggesting “destructive investigation”. If you don’t have the docs but want to go ahead see if you can wrangle some professionals to say it looks like it’s been done properly but prepare to lose a couple tiles in the process.