While any muzzle loader that is capable of shooting sabots well can be tuned with a 200gr 40 cal bullet and 90 or 100gr of powder to work well for the smaller lighter people I believe that a Triumph Bone Collector or a buggy rifle which has the shorter trigger pull length would give them a much better chance of hitting what they shoot at. If you know how to measure trigger pull and cut down a wood stock you might fit an omega with a wood stock to her.
To measure the trigger pull length hold your elbow strait out from your shoulder with your palm turned to ward your face and measure from the right angle that your elbow makes to the joint on your trigger finger closest to the finger nail. This should be very close to the measurement you get from the trigger to the center of the butt plate or recoil pad. Not fitting the weight of the gun or trigger pull right is where a lot of young people and ladies get discouraged because trying to shoot a gun that don't fit and learn to shoot at the same time is a mountain to climb. There may be other guns with the right trigger pull and weight but the only ones I can think of right now that are any good in the sense that I would give them to my wife or kids would have to be altered. A lot of people do not realize how important this is; I do I enjoyed shooting muzzleloaders and archery with my first wife until a few months before she died about 45 years and while I love my present wife very much I miss the companionship of shooting and hunting together.