I echo what Laura said although we got a silk duvet insert that is 100% silk...it is not cheap, but feels like a butterfly on you and is incredibly lightweight, yet warm enough when you need it. Silk is naturally anti-everything, so allergies are reduced if you have them. We don't put the insert in anything, we just use it directly as the cover and wash delicately as needed.

I have found that no matter what you use to cover, though, it gets stuck under you, twisted, etc. You might want to consider getting your mom silk jammies so that she doesn't even need a cover, depending on where you live, of course.

We pair the butterfly cover with "fat bastard", a particularly thick pillow we use for body support. It makes me laugh to think that we named our bedding in general, and that one part is lightweight and beaucolic in name while the other is heavy and profane! I'll take the laughs wherever I can get them these days