This was an easy repair! The zipper lost its pulling part, so I just replaced it with another one I had on hand.
I had to take off the slider stopper thing on the top with a plier. (Yes, I yanked it off.) Then, I had to make a cluster of read thread where the slider stopper was, and I put a few drops of UV resin to make the cluster of thread look like a slider stopper. I think it worked. The best thing is that the owner of this down filled jacket can get more use out of it.