What are the options for replacing a claw foot tub with a pre-fab shower stall?

My parents own an 1890 home in rural Nevada. They need to replace the clic claw foot tub with a shower. Their local plumber stated that custom 'walls' needed to be contstructed to hold the pre-fab shower stall. I've seen many shower configurations with 3 gl walls and a back wall. They would like to acquire a pre-fab shower stall with clear/gl walls and put it where the tub is (against a wall back). Flooring is stable. The shower stall could plug into the tub plumbing. Why would the plumber need to create side walls in this situation? What are the options? Thanks.