She must stay behind you.
It was narrated from Anas (ra) that his grandmother Mulaykah invited the Messenger of Allah (salla Allahu alahi wa sallam) to eat some food that she had made. He ate some of it, then he said: “Get up and I will lead you in prayer.” Anas said: I went to a reed mat of ours that had turned black from long use, and sprinkled it with water, then the Messenger of Allah (salla Allahu alahi wa sallam) stood on it, and the orphan and I stood behind him, and the old lady behind us, and the Messenger of Allah (salla Allahu alahi wa sallam) led us in praying two rak‘ahs, then he left..
(Bukhary and Muslim)