I doubt all you fools. It looks like an open shade to me plus post to brighten up the image. Plus, doesn't look like there was any real attention to the lighting situation. I think it's brightly lit... but very poorly lit. So Liam - Knowing what she likes about that photo, I think you can do better with some prep work.
As others have implicated, it seems like the backlighting is what she wants. That said - with one flash and no information about whether you're able to trigger the flash off-camera, I think you'll have to judge the situation and get the setting sun behind her if you can or if there's another source of light.
Are you going to be able to get that flash off camera? You'll need a cord or a wireless trigger.
If you can, then a softbox will help light her face. I'd place it camera left or right on the same side that the sun is setting. I think the easiest approach is to expose for the background, pop the flash on ETTL and FEC + or - as necessary to light her.
If you can manage to match the background exposure with her face, then you can place the light behind her... I'd do it bare, without the softbox, to create some backlighting. Test a few on manual flash settings.
That's how I'd do it. Not saying it's the right way... but that's my way... lol.
Can't read the entire link in the image Liam posted... looks like AmandaM***photography.com. *Edit - Durr... it's at the top... AmandaMcPhee... going to check out her site now.