You say that your friend “was, and I believe still is, Catholic,” but in the very next sentence say “Some years ago she converted to Islam and married a Muslim.” I point out this contradiction because it is important in answering your question. If your friend never became Muslim, then she need not “return” to the Church, as she never left to begin with. But if she did convert to Islam, then she should go to confession and confess that she left the Church by converting to Islam. Once absolved, she would be once again restored to full communion with the Church. If she has not received some of the initiation sacraments – for a baptized Catholic, that would be first confession, first Communion, and confirmation – then she should ask the priest for instruction in the faith and preparation to complete the initiation sacraments.
