It comes from "women shall keep silent in the church" which is basically interpreted as women should not preach or teach Christian men (men being adults... not children's Sunday School). I believe this goes for "leadership roles" such as pastors. Obviously this verse leads to a great deal of controversy, but then lots of things that we don't "agree" with do. If you take the verse literally, then women should never speak in a church. So since no one believes that is the case, it is taken as "leadership". In an independent-fundamental Baptist church, a woman cannot even direct the choir (however, I did exactly that in the church's Christian school.) When there is a woman at the pulpit for 40 minutes, she is a "guest speaker" not a preacher. It's all in the way you word it, I guess.
Is she teaching from God's Word? This is in your church?