For whatever it's worth, I use a Mountain Smith Cairn II fanny pack on my workout hikes. It's a large fanny pack with excellent, tough zippers and a wide belt with not one, but two delta straps for heavier loads.
It carries not only my SIG 220, but a bottle of water, extra clothing, a power bar, and my reading glasses, etc.
The only thing awkward with it is getting to the gun. One needs to practice that, make sure the gun is placed correctly in the pack and make sure the zippers are easy to reach. But it can be done. I wouldn't use it if I wasn't confident I could use it effectively.
My suggestion is have her try on some found in back packing stores, have her decide what she needs and can use. E
