CVA Bobcat
This might be the answer to your problems. A CVA Bobcat is a .50 caliber sidelock rifle with a 1:48 twist. They generally shoot round ball very well and conicals also. You could load this with about 65 grains of Goex FFg and a patched round ball and keep her shots under 75 yards and she should be fine. You could also remove the back butt plate and fill the hollow plastic stock with some lead and construction foam to add some weight and balance the rifle better and take the recoil out of it if she is recoil sensative.
I have a Mountain Stalker in .54 caliber which is made by CVA and is the same thing as the Bobcat. I was shooting mine today in the yard with 80 grains of Goex FFFg and a patched round ball. This is not a load you would want your wife to shoot. These rifles are light and they kick a little if you get the powder charge too high.
They are a light (which is why I bought it... for stalking) accurate rifle. I took three deer last year with this rifle shooting round ball. I did throw a cheap set of fiber optic sights on it. This is an excellent shooting little rifle actually. Although mine does not seem to like conicals but with round ball it will tear a bulls eye out at 50 yards all day.
Another option is to go to some of the on line auction sites and look for a .50 caliber CVA Hawkins. Some of them can be picked up for a little over $100.00. They are heavier so the recoil is not there, and are normally good shooting guns that function well. The only thing is your buying a used gun. Although I have bought a number of auction rifles and so far all of them have been excellent.
If you went with an inline, and loaded a 200 grain SWC and 90 grains of Goex FFg the recoil would not be too bad. I have a Staghorn that I shoot that load out of when I want to plink. I am sure that it would take a deer still but actually I would prefer a larger projectile....
You could always load some of them down to 70 grains (which has no recoil) and see what kind of accuracy you would get. He is a site where you can look at some cheap projectiles like round ball, etc...
Black Jack's bullets
Your inlines might shoot round ball by the way. I shoot round ball out of my Knight Wolverine II with good accuracy. I would not shoot 100 yards with it, but 50 would be no problem. The trick with the knight is to drop the powder charge low, so the patch is not pushed to fast causing it to skip the rifling.....