Each case is individual and requires a particular judgment about what is the best or most charitable thing to do. There are a lot of factors to be weighed and questions to be answered before a person makes a decision to attend the marriage of a Catholic marrying outside the Church.
Among the questions to be answered are: Will my friend or child or relative take my attendance as approval of what he or she is doing? Will my decision to attend encourage the person to go ahead and marry contrary to God’s law and enter a sinful union? Or does the person know and realize my attendance will only be a sign of caring and friendship?
If I don’t attend, will it only embitter that person and drive him or her further from the Church, thus making any future effort to be reconciled with the Church and God harder and more unlikely? Will failing to attend cause divisions and enmity in the family? What action—attending or not attending—is more likely to keep open communication with the person and, in the end, have the most beneficial spiritual effect?
That is one set of considerations. Another is: What effect will my attending or not attending the wedding have on other people, especially those for whom I have the most responsibility? If I attend, will my children or other family members take it to mean that I see nothing wrong with what the other person is doing?
The closer the relationship, of course, the more the pressure to attend and the more significant the act of attending or not attending.