Feathering an engine

I am an historian who is writing a nonfiction book involving WWII B-24 airmen. I have done considerable research on B-24s, but I am not a pilot. In my book, I tell of an incident in which an engine died and was feathered. I have been repeatedly told, and have repeatedly read, that feathering is done to reduce drag generated by a propeller in a dead engine. But a pilot with whom I was conversing said that he believed that part of the problem in a dead, unfeathered engine is the windmilling driving the dead engine, not just the angle of the prop.

Can anyone clarify this? Does one feather an engine simply to reduce the drag generated by the angle of the propeller, or it is also to stop the engine from turning? If the latter is correct, why?

Thanks to everyone.