I'm surprised no one else mentioned it, but I'd get her a vegetable cleaver.
It's exactly what it sounds like, a cleaver for vegetables.
A lot of the problem she's having with chopping vegetables, especially sweet potatoes, is probably wedging of the knife blade and poor technique.
The added size and heft of the veggie cleaver along with a well designed grind does a lot to overcome both issues. As long as you use appropriate cutting boards and don't start hacking at ribs with it it's not likely to need much sharpening.
I'm looking for a suitable model at the moment and will update later. I have one but it's not exactly a budget option and it's a high carbon steel.
UPDATE:
That's a CCK Small Stainless Cleaver. I don't know if the retailer is a member here or not so I won't mention them specifically, but a quick Google search should pull it up. $70 where I found it.
I know they look odd to us westerners, but they're wonderful. Slice and chop like a dream and the width allows you to scoop and move the chopped veggies with ease.