My wife and I have separate accounts on our iMac with me being the Administrator - OS High Sierra 10.13.6. In my wife's account, the Downloads folder is not in the Dock.

This morning she tried to view a credit card statement while at the credit card site. However, the round download icon did not go to a Downloads folder but rather shot off the top right corner of the screen. I assume that by now it has landed in a farmer's field somewhere in France.

In my administrator account, I viewed her User account and it shows a Downloads folder with a bunch of stuff in it - but not the missing credit card statement - so I assume the folder did once exist. I also assume my wife may have accidentally dragged the Downloads folder off the Dock.

How do I restore it?

Assuming that your diagnosis is correct, go to Users/her account. There should be a Downloads folder there. Drag it into the dock. Of course, you should be logged into her account to do this.

Done....thanks.