Firstly, just make sure you understand the disorder as best as possible. Never try to push or force her to talk. It isn't in her mind. She physically cannot speak, so it is unwise to push her anxiety higher. More information on the disorder can be found here.
Long and short, there is nothing you can do to help her other than general anxiety treatments. Just supporting her can be enough and if you seek professional treatment, she will eventually outgrow it. Left untreated, she may progress into adulthood.
I once knew a girl with selective mutism. She could only talk to her parents.
When we were 11, we played on the same softball team and I only heard her voice once. (She was speaking to her father, not to me.)
Her father told me that the best thing I could do was help her. Don't force her to talk, just ask to be her partner when it comes to partner work, etc.
This is different, of course, when you're the parent, but pretty much same goes. Seek treatment, and be her biggest advocate. I know its hard to have to sit on the sidelines!
Also: Educate those around you of her disorder and share the link with them, that I shared with you (Her friends, teachers, etc.) Awareness is key.
I'm purposely being vague on the meaning of treatment, and that's because treatments aren't usually supported on a widespread scale (due to this disorder not getting a ton of awareness.) In fact, there's really only one wide-researched and approved treatment,which is Cognitive Behavioral Therapy. So, not a whole lot of choices.
The last time I saw the girl I once knew was around age 15. Her parents hadn't yet sought treatments, and I doubt they were planning to. She still wasn't able talk.