My area of expertise!
I don't know what kind of camera your wife has, but I'll assume it's a heavy DSLR for argument's sake.
Here's what I've found:
1) The best waist pack is the Sprint from Clik Elite. The sprint is very stable. However, there are two kinds of waistpacks- small ones I can wear under a backpack and large ones I can't, and this is the latter. if she has a backpack, the Sprint probably won't work.
2) The best system I have is to use two carabiners to attach a LowePro Toploader AW 45 or AW 55. Now, Lowepro makes two different kinds, the cheap "regulars" and the "Pro" line. You want the REGULARS, not the pros. The extra features come at literally 2x the weight, and the complexity of the lid means the weather shield is uselessly complicated. Having a chest loader is actually WAY better than a waistpack for me, because it encourages me to shoot more since I'm not constantly adjusting a waist pack that rubs on a backpack.
3) Now, if you're day hiking and carrying nothing, there's one more option: The Clik Elite Access is a version of the Sprint that attaches on the chest, so it's the most stable and the most convenient and the safest (since, by instinct, your arms shoot forwards if you fall down). This is what I would use if it attached to my backpack straps, which it doesn't.
4) I haven't liked mountainsmith as much as anything else. I wouldn't dive in. The cases at REI that I like are the Lowepro Outback, lowepro AW 55, and the Pacsafe Venture V8, although that last one is heavy as a truck. These would all be chest packs, though. The Clik Elite stuff is so good when it comes to waist packs, it might be worth the risk of having to pay return shipping to Amazon or Clik Elite themselves.
Caveat: I am a guy, and not a woman, so without being too graphic, anatomy might mean the Sprint is the way to go. Clik Elite has larger storage waist packs, too, which would allow you to carry the ten essentials and a raincoat next to your DSLR< or an extra lens. Check them out!