Fitting women for ball weight is tough, because they really don't seem to want to put any effort into delivering the ball. I've seen women carry a toddler, purse and a couple bags of groceries but can't manage a 12lb ball. Go figure.
But since you say your wife has some problems associated with her wrist, it might be better off to start with a lower weight. Keep in mind that a ball that fits properly allows the person to handle more weight then it does if they use a house ball. House balls never fit right and always have thumb holes that are too big and too round. So have her fitted, I'd go conventional grip, and maybe 12 lbs. Because you say that she can handle a 10 lb house ball. The wrist brace will help with her physical problems, as will the conventional grip vs finger tip. Then if she gets really comfortable (after a while) with the weight and no strain on her wrist, then think about plugging the fingers to go fingertip. Get a Tropical Storm. That seems to be the best entry level ball I've seen perform.
The old Robbys wrist brace with the bar on the palm and top of the hand should be enough to give her support to her wrist.