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This is for my friend.

Her mother stayed here for 2 years to help her take care of her baby. Her mother did not plan to come back anymore and so did not do extension. Her mother came back to China ~1 year ago, and my friend wants her mother to come back for another half a year or so. Can her mother still get visa approved? Or her mother has to wait until staying 2 years in China?

-Depends on how long her mother oeverstayed in the United States last time (i.e., from her B2 I-94 expriation date to the date she left). If her overstay was more than 6 months and less than one year, then she will be barred to enter US for 3 years; if her overstay was more than 1 year, then she will be barred for 10 years. 

Of course,  her mother can apply for waiver if she wants to remove the bar and come to US earlier. See this link:

Immigrant and Non-Immigrant Visa (NIV) Waivers