So much of Catholic moral teaching and theology is clear to me. However, issues of marriage seem to get murky. I have a friend who is a non-practicing Baptist. She is marrying a baptized Roman Catholic who has not seen the inside of a church since his baptism. They are being married by a Catholic priest who has left his diocese in favor of marriage and is (most likely) illicitly witnessing marriages. Sorry if this is a stupid question, but can my wife and I attend?
If your friend’s fiance was never raised in the Catholic faith, he may not consider himself to be a Catholic. If not, then he is not formally bound by the Catholic form of marriage. If that is the case, then the wedding would basically be a civil wedding and your only concern would be whether or not their minister is licensed by the state to witness marriages. If he is (and so long as he is only acting in a civil capacity and not illicitly as a Catholic priest), and if there are no other obvious impediments to the marriage, the wedding would be presumed by the Church to be valid and sacramental because it would be considered the civil wedding of two baptized non-Catholics.
However, if your friend’s fiance is a lapsed Catholic (i.e., he considers himself a Catholic even though he does not practice), then he is bound by the Catholic form of marriage and must obtain dispensations from his bishop to marry a non-Catholic in a non-Catholic ceremony. He also could not have a laicized Catholic priest witness the marriage (even assuming their minister was properly laicized and didn’t just “leave”). Without the proper dispensations, and with a laicized Catholic priest witnessing the marriage, the Church would presume such a marriage to be invalid and non-sacramental (although it would be presumed legal if the state recognizes the marriage).
That said, the Church does not explicitly forbid Catholics from attending irregular marriages. Catholics must use their prudential judgment to both uphold the Catholic understanding of marriage and to determine what action would be best in this situation given the unique dynamics at play here.