I guess the key things are don't complete her sentences for her. You might think you're helping, but you're not. Also, you shouldn't have a rushing body language so avoid doing things such as tapping or looking at your watch. You don't have to do this, but I find it helps when the person speaks at a slower pace. It just gives the impression that you're not in a rush. Of course you shouldn't make it comically slow if you're going to try this. Talking to a person that stutter really isn't that hard IMO. You just have to show them you are patient so they can chill out and take their time when they talk. And it's stupid easy to gauge how well you are doing. If she's stuttering more, you're doing something wrong cause you're making her feel rushed and nervous. If she's stuttering less, you're doing something right. Easy peasy.