We have an old "feather bed" or "eider down", it is absolutely ancient and is currently stored away. If she is talking about the same thing - it is an old kind of duvet (ours dates from the 1920s), the down (usually duck) is stuffed into channels, the fabric containing the feathers is usually very dense and I do believe it was waxed or treated some way to prevent the pin feathers poking through. The whole thing was then encased in a satin/sateen cover which was then stitched on. Personally I don't like them and the same goes for feather pillows, they tend to clump and are not great for people with allergies. They are murder to wash and you have to keep tossing and shaking them about as they dry otherwise the feathers clog together.
I doubt these days anyone sells the feathers to "do it yourself"...they would be a nightmare to control and absolutely no help to your respiratory system.