If she is a good friend, ask her to look at her nose, then raise her eyes and look at the space above it - I.e. the point right in from of her eyes. Then ask her to slowly move her focus to some object to the left of your camera. Probably just half a meter left no more than that. It is not difficult at all after a minute of training. Put on cont. shooting and keep shooting while she is doing it. It is fun so she will be smiling if not laughing. No guarantee, but you can get lucky with one or two shots.
I never tried that, but that is what I would do if the girl insisted on her eyes looking straight. It is very personal and some may look utterly stupid while doing this methinks. So take it just as a suggestion.