WTF? A bag of powdered cement?

Make sure to get a second opinion just in case (10-15K is nothing to sniff at!). Some photos from a snake cam would be good too. More information will empower you.

N.B. "Tiles need to be redone" is an understatement. A proper job will involve ripping ALL tiling from the bathroom, removing all adhesives (may be easy or hard depending on what they used), probably re-rendering, re-waterproofing (ideally twice), then laying new tiles. Cheaping out on these steps can lead to earlier ones ruining later ones, requiring you to do the entire thing all over again (it's a horror).