I asked about a similar scenario while in the seminary. According to the priest, any baptized Christian can go to the confessional and still, the priest is bound to keep the seal. Although the girl is not a Catholic, she went to the confessional with it’s Catholic understanding and again whatever is said in the confessional is subject to the seal. If the priest wanted to make a distinction then he should have informed the penitent that he would not be hearing her confession. It would have also helped if the priest had prudently asked her to set up an appointment to discuss her issues. However, the priest heard the girl’s confession which she took as her opportunity to speak with God and confess her sins. If all of the above did occur, then she may have a case to ask for canonical penalties upon the priest, which an intentional breaking of the seal would be automatic excommunication. However, it would also depend on how the diocesan bishop and tribunal interpret the case.
Your priest was right:
Can. 844 §1. Catholic ministers administer the sacraments licitly to Catholic members of the Christian faithful alone, who likewise receive them licitly from Catholic ministers alone, without prejudice to the prescripts of §§2, 3, and 4 of this canon, and ⇒ can. 861, §2.
§2. Whenever necessity requires it or true spiritual advantage suggests it, and provided that danger of error or of indifferentism is avoided, the Christian faithful for whom it is physically or morally impossible to approach a Catholic minister are permitted to receive the sacraments of penance, Eucharist, and anointing of the sick from non-
Catholic ministers in whose Churches these sacraments are valid.
§3. Catholic ministers administer the sacraments of penance, Eucharist, and anointing of the sick licitly to members of Eastern Churches which do not have full communion with the Catholic Church if they seek such on their own accord and are properly disposed. This is also valid for members of other Churches which in the judgment of the Apostolic See are in the same condition in regard to the sacraments as these Eastern Churches.
§4. If the danger of death is present or if, in the judgment of the diocesan bishop or conference of bishops, some other grave necessity urges it, Catholic ministers administer these same sacraments licitly also to other Christians not having full communion with the Catholic Church, who cannot approach a minister of their own community and who seek such on their own accord, provided that they manifest Catholic faith in respect to these sacraments and are properly disposed.
§5. For the cases mentioned in §§2, 3, and 4, the diocesan bishop or conference of bishops is not to issue general norms except after consultation at least with the local competent authority of the interested non-Catholic Church or community.
The confession would still be valid, it seems, just illicit. The question is whether the illicitness of the confession invalidated the seal of the confession. I would think not (kind of like illicit baptism being still valid baptism) since there’s no mention of licitness in the section on the seal.
Can. 983 §1. The sacramental seal is inviolable; therefore it is absolutely forbidden for a confessor to betray in any way a penitent in words or in any manner and for any reason.
I would want to see the article on the priest and hear his side, too, since media LOVES making the Church and priests look bad.