Good advice here already but if you're looking to get a portrait of your friend, rather than an environmental portrait, then fill the frame with her a bit more.
With the lens you're using you can pick any length you want and you won't go far wrong - it's not short enough to give an unflattering perspective. I'd go as long as you can without the aperture closing down too much and introducing the problem of camera shake. Having said that, if you're happy working that way you can always stick it on a tripod.
Out in the woods you probably have room to separate your subject from the background by physically moving her away from it - so working with your lens wide open, even if that only means f5.6, should be fine.
Finally, if you're close enough, and this'll be controversial , I'd use that oft-maligned pop-up flash, set to under-expose a stop or two (depending on the light levels), just to fill the shadows. Pop-up flash is often terrible when used as a significant proportion of the illumination of a subject but it can be fine just to add a quick, hassle-free, fill-in to get rid of some of the worst contrast.
Looking forward to seeing how you get on.