Look in the front of the missalette. There should be a statement there about who is invited to Communion. Point it out to her gently. If she objects, remind her that she is a guest and just as you would follow her rules when visiting her house, she should follow our rules when visiting one of our churches. Exception: If she is an Orthodox Christian, she is free, from our perspective, (though probably not from her church's perspective) to receive Communion on her own initiative.
If she doesn't understand why we don't admit all Christians to Holy Communion, at an appropriate time — probably after the liturgy — you can explain that historically and doctrinally, we see communion as a way of recognizing those who are fully united to our Church — to be in communion with the Catholic Church is not only to be invited to receive Communion, it is to be fully Catholic.
Those who are not in communion with the Catholic Church are not Catholic, and vice versa; likewise, those who are not in communion with the Catholic Church cannot receive communion in the Catholic Church, and vice versa. We see it as a visible sign of unity with the Church. Thus we do not, as Protestants typically do, practice open communion because it means something different to us. It's an act of intimacy with the Church, meant only to be shared by her members.
There are certain exceptions to this norm but you probably don't want to get into that with her.