-Wheh she enters US using the immigrant visa, the IO at the port of entry will stamp the immigrant visa (i.e. endorse it). This endorsed/stamped visa is equal to the temporary green card which is valid for one year. Then, USCIS will send her the plastic 10-year green card to her about one month later.
If she leaves US before she gets the plastic green card and she will return to US within one year (she had better return within 6 months), she just uses the endored immigrant visa to come back. If she plans to stay in China for more than one year, she needs to apply for reentry permit to travel.