cut into hardiebacker wall and slide shower tray in?

Hello all,
we’ve been having an ongoing saga with our loft bathroom. The new stud walls were built out of square by the loft company. The corner walls are more than 90 degrees so when the shower tray is pushed into corner there is a 15mm gap between the tray and the wall on each side by the time you get to the outer edge.
We had the ply walls replaced with hardiebacker and thought the tiler would pack out the walls to make square again but he didn’t.
He then cut a slot into the hardiebacker…

cut into hardiebacker wall and slide shower tray in?