Visa for close friend of family

I have a working permit in China and my wife and baby accompany me.
Question: a close freind of the family was helping us to take care of our baby since it was born. What would be the Visa suggested to have her stay with us in China for the longest possible time ? She is Vietnamese living in Vietnam. In this case what is the longest possible stay for her in China ? Does she have to take a L-Visa for 3 months and then prolong 2 month before exiting and applying for another Visa ? Thanks so much !