-She has no problem to enter U.S and get another I-94 period (usually for another six months) as long as she returns to U.S. before the visa expires. Many people go to Mexico/Canada to do the same way to get another period for staying in U.S. So do not worry about that.
Does she need to stay longer than 6 months and re-apply visa?
-She may be difficult to get a visa again in such short period even if she stays for about six months in China.