In a large mixing bowl, beat together the sweetened condensed milk and thawed lemonade concentrate until completely smooth.

Add the lemon instant pudding mix and continue mixing until the filling begins to thicken.

Gently fold in the thawed Cool Whip until the mixture becomes light, fluffy, and fully combined.

Spoon the filling evenly into the prepared graham cracker crust and smooth the top.

Refrigerate for at least 4 hours, or until firm enough to slice. For the creamiest texture, chill overnight.

Just before serving, top each slice with Reddi-wip if desired.