Dont know if they make reduced velo Foster slug loads but if they do try them. In a full choke Id be reluctant to use the Brenneke style, though they have deep vanes that should compress going through it, they are kinda expensive and I got better groups with basic Win. Foster's in mine.
Get a couple of boxes of Rem., Fed., and Win. and try them all.
For her I suggest standing, not off the bench or it will beat her up, then game over. My shooting experienced wife only took 3 shots before she gave up on the slug gun.
Dont let her get her cheek down on the comb either, but use more heads up sighting. That thing will punch her in the cheekbone and sighting right down the top of the barrel wont likely put the slug on target. You might want to do the tests and get the sighting figured first. a little wood block simulating a rear sight taped around the barrel and Magic Marker blacked may give her the right heads up and reference point to make good hits.
BTW, do you want it to be a choked gun for shot use? Even so, will you use shot to distance or how would a cyl. bore 20 be for you guys for closer in/more open pattern shot loads?