Gonna have to start by draining pool down 6 inches or so below that return, remove it and see what you're dealing with. I'd fashion at least a 8x8 thin metal plate for both sides, bigger depending how far out you need to actually remove the totally corroded part. Sand it, rustoleum it, then pop rivet, gorilla glue and then 3 layers of duct tape around edges on inside of pool wall. Gonna have to remove a couple of top rails and pool wall rails to access the inside.