1/8" Stainless Steel Plate
IMO, your best bet would be to buy a big bag target, like the Morrell Outdoor Bag Target, and start her off shooting under ten yards. Have her shoot there for a while and get the basic mechanics down, start developing a good anchor point, and then start backing her up a little at a time. The Morrell Outdoor Bag Targets are 3ft X 3ft, and if you start her off close, there's almost no way she can miss that target. If you teach her good mechanics, she can build her confidence without having to focus hard on hitting the target, then once she is comfortable, she can start backing up and working her way to 20.
My wife started shooting two years ago, and right off the bat, she was wanting to shoot 20y like everyone else was, but I wouldn't let her. We worked on things at close range for a long time before I let her back up, and now she shoots very well out to 30y.
Good luck with introducing your wife to archery!