Reference: http://www.alfawzan.af.org.sa/node/14854
Question: What is the Sunnah regarding [the manner of] the prayer of the woman with her husband? Should she stand in line by his side, or should she pray behind him?
Response: They are both the same; there is no harm if she was to stand in line by his side, [just] as there is no harm in her standing in line behind him.