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I suggest her to file I-539 for B-2 extension and then, she can leave US any time after the I-539 filing. I-539 can be filed electronically and get the receipt instantly. Keep the I-539 receipt for future B-2 visa application.
See these website for I-539 electronic filing:
http://www.uscis.gov/uscis-elis/e-filing-using-uscis-elis/e-filing-form-i-539-using-uscis-elis
http://www.us-immigration.com/blog/how-to-file-your-i-539-form-online