All you need is a single AR mag pouch with a coupled LAR mag, and a shotgun shell caddy of some sort.
So long as you can get 8 shotgun shells on your existing IPSC belt, you're good to go.
The money you spend on the chest rig alone will buy you nice high quality shotgun shell caddies, which, dollars to donuts, are the best possible investment for 3Gun. That's what it all comes down to.
You already have the belt as I understand it? Spend the $100 on the right accessories for the belts, and you'll be laughing. Seriously. Pick up a blade tek AR mag pouch and something for the shotgun shells. APC, Carbon Arms, or Invictus practical, and you're off to the races. If you really want, you can get the blade tek double AR mag pouch which will hold two of your ideal AR magazines in the space of one.
That's all you need to get started. Already having the IPSC belt is like 60% of the way there.
You only use a chest rig these days if you're doing CQB. Mine just hangs on the wall these days as a testament to something I used to do. Now with the belt and tek loks, I can just add or delete stuff from my belt as I need it.
You come to the forum for advice. The advice here is use a belt, which she already has. She needs two accessories for her belt to be ready to go.
Chest rigs are a step in the wrong direction for three gun.