Countouring should work fine on a round face, you just need to use a fairly matte colour that's not an awful lot darker than her skin, and you need to blend lots...
I have a round face and do light contouring every day and it makes a world of difference to how I feel and how I look in photographs. I start by sucking in my cheeks (ahhh, the world-famous "fishy face"
and blend a bit of colour in the bits under where the cheek sinks in (I would say under the cheekbones, but I don't really have any to speak of
Next, I blend the colour down and round onto underneath my chin (I have a slight double chin and this does help a lot by shadowing it somewhat), and up on each side, so that I shade the edge of my face outside where the eye is, and the outer third of each side of my forehead. Using sheer but not shiny colour all over the face like this, and really blending it in, stops the "cheekbone" shadow area looking false. I'm NW20 and the colour I use is a pale toffee-coloured blush with a tiny touch of rose pink to it (one of the d/c cheekhues, can't remember the shade name)
Next I do blush: I smile for this, and I use a complimenting but brighter colour - usually a rose pink, I'm a big fan of Dollymix - but this time I use a product with a slight sheen. Concentrate the blush on your cheek "apple", but blend a bit round softly round in a "C" shape in each side across your cheekbone, and curling round to stop just where the outside of your eyebrow is.... I think of this as "following the line of the smile" if you see what I mean.... again, blending is the key.... with the blush part what you're trying to do is emulate where you would naturally flush, so bear this in mind.
After this, I pop a bit of highlight on, something sheer and shimmery (Pink Opal pig or Lightscapade are my faves) just across the cheekbone and a teeny little bit on the outside of my under-eyebrow area.
This is all kinda hard to describe textually, but what you're trying to do is create the illusion of shadow in areas you're trying to minimise (with the contour shade), and add extra light (with the highlight) to areas where the light would naturally fall (brow bones, cheekbones) more prominently on a narrower face. I would suggest you play around with how a few things look on her, and try taking some test photos to see how it all turns out.
HTH, I'm no expert, it's just what works best for me.
