That's a vertical crack in a structural wall. It has to be repaired from the inside of the pool because you have no idea how far down that crack goes. From the inside of the pool, that crack could run all the way down the wall, along the floor, and then up the other side. There's no way to know without chipping out the plaster and inspecting. If it's just that section of wall, then they can notch the crack, add structural staples and fill with epoxy and hydraulic cement and then redo the plaster.
What you have is potentially a major amount of structural reworking. It expensive but the shell should be under warranty from the gunite company that did the work as well as the PB's insurance. This is not on you. But you may want to negotiate with the PB to see what is covered under their warranty and what you might be willing to pay for out of pocket.
The solution you want is for the crack to be fixed from the inside, not the outside. How you arrive at that preferred solution with the builder will take negotiating.
@AQUA~HOLICS might have more insight.