Easton Jazz or Platinums aluminums are light enough and spined lightly enough. You can get them spined about 1000 which would be about right for your wife. Don't cut them down to her draw length--leave them pretty long.
Another good option is the 28" wood youth arrows from 3 Rivers Archery, except I recommend cutting off the crimp-on points, re-tapering, and gluing-on 70 grn 5/16" field points.
The aluminums will out-perform the woodies (fly like darts) but cost a lot more. At only 20 lbs, you really need to keep the weight down on the arrows to keep your wife from getting discouraged.
Good luck!
~Brett