Apparently, she did not just start going, but the priest approved of this and has told her it is OK.
I would talk privately to the priest.
In the 1983 Code of Canon Law the following is prescribed:
*1. Catholic ministers may licitly administer the sacraments to Catholic members of the Christian faithful only and, likewise, the latter may licitly receive the sacraments only from Catholic ministers with due regard for parts 2, 3, and 4 of this canon, and can. 861, part 2.
Whenever necessity requires or genuine spiritual advantage suggests, and provided that the danger of error or indifferentism is avoided, it is lawful for the faithful for whom it is physically or morally impossible to approach a Catholic minister, to receive the sacraments of penance, Eucharist, and anointing of the sick from non-Catholic ministers in whose churches these sacraments are valid.
Catholic ministers may licitly administer the sacraments of penance, Eucharist and anointing of the sick to members of the oriental churches which do not have full communion with the Catholic Church, if they ask on their own for the sacraments and are properly disposed. This holds also for members of other churches, which in the judgment of the Apostolic See are in the same condition as the oriental churches as far as these sacraments are concerned.
If the danger of death is present or other grave necessity, in the judgment of the diocesan bishop or the conference of bishops, Catholic ministers may licitly administer these sacraments to other Christians who do not have full communion with the Catholic Church, who cannot approach a minister of their own community and on their own ask for it, provided they manifest Catholic faith in these sacraments and are properly disposed.
For the cases in parts 2, 3, and 4, neither the diocesan bishop nor the conference of bishops is to enact general norms except after consultation with at least the local competent authority of the interested non- Catholic Church or community.*
This canon reserves the sacraments to Catholics, that is, those who are in communion with the Church. If this organist is “in communion with the Church”, i.e. she believes and holds all that the Church teaches to be true, and believes the CC is the One, True Church, then she should ask to be received into the Church.
If not, she is not in communion with the Church and should, therefore, not receive the Eucharist.