You could try winnowing it. Is he putting other plastic in the shredder, like envelopes with plastic windows, or other paper types not really suitable for worm composting?

I do find what I would assume are shredded up windows from envelopes and charge cards. I do not find glossy looking things. What is winnowing? I never heard that term before.

Winnowing is lofting the stuff into the air in a breeze to let the light stuff float off and the heavy stuff to fall. Often done with wheat collection. What you would be doing is catching the floating stuff and tossing the heavier plastic. If there's plastic coated cardboard cards getting shredded, those will still float but shouldn't be bedded for the worms so much. If you are seeing plastic wondow stuff in there that will float and you will need to go back through your paper and pick all of it out. You don't want little shreds of plastic in your compost.

I realize it's not really my buisness but I'm curious. How many charge cards is your dad using up on the regular that they are so often in the shred bin? Once in a while is a thing, but if it's on the regular... maybe a solution is to ask him to sort his shredding. Bedding safe paper in a pile, the rest in another. Shred for the worms, then shred the other stuff seperate.