Make sure that your whipped topping is completely thawed (by placing it in the refrigerator overnight). Also make sure that your pineapple is very well drained. I like to put the pineapple in a mesh colander and then squeeze it dry with a towel.
In a large bowl, stir together condensed milk, cherry pie filling, and drained crushed pineapple. Gently fold in the whipped topping just until combined.
Cover and refrigerate until ready to serve.
