Please don't do this, you're asking for problems. If you see your contractor do this STOP HIM !
Here is how I do the wallboard...
   • Proper Way To Set WALLBOARD For Shower  

FOR THE ALUMINUM EDGING CLICK BELOW
https://www.amazon.com/gp/product/B00...

FOR THE DRAIN YOU SEE CLICK BELOW
https://www.amazon.com/gp/product/B07...