The post above captures the point pretty well.
Since this is a matter involving discretion, the success will depend on how well your friend prepares her case and the consular officer who reviews her case.
However, to prepare her case, there are two key points: it is in the interest of the citizen child to have her to come to the States. First, she will want to establish that the citizen child is coming back to the States for good (not just come back so the child can bring her mother along with him/her). For example, the child is coming back to receive an education or settle down.
Second, it is necessary for her to accompany the child. For example, the child may not have any family member to take care of him/her. Or the child is extremely young and therefore needs the mother.
Check out the INS site for a form specifically dealing with this kind of petition. It lists out all the details.
But, it is important for her to know that her petition to come the States will depend the test of how to best protect the welfare of the citizen child. She herself does not have a claim to be here. She, therefore, should prepare her case accordingly. It is a lot more difficult when the child is in China and not currently in the States now.