If your daughter is a former Catholic (i.e., she has left the faith), she is not formally bound by Catholic marital law. Assuming that there are no obvious impediments to the marriage (e.g., previous marriage, close blood relationship), the Church would presume the marriage to be valid. If both parties are baptized, the Church would presume the marriage to be sacramental.
However, if your daughter is a lapsed Catholic – as there are indications from your account that she is – then she is bound by Catholic marital law. She would need to obtain from her local bishop a dispensation from cult to marry a non-Catholic and a dispensation from form to marry in a non-Catholic ritual. These are fairly routine procedures that her local parish can obtain for her on her behalf. If such dispensations are granted, the marriage would be presumed valid (sacramental, if both are baptized Christians).
In both such cases outlined above, you would be free to attend and participate in your daughter’s wedding. But let’s say that she is a lapsed Catholic who refuses to obtain the proper dispensations. The marriage would be presumed invalid, but could you attend anyway?
The Church has not officially stated that Catholics may not attend invalid marriages, leaving that decision to the prudential judgment of those involved according to the individual dynamics of the situation. The Church expects Catholics to make the best decision they can in accordance to the Catholic understanding of the sanctity of marriage.
If you believed that your daughter was doing the best that she could, according to the limited knowledge she has of Church teaching, to do the right thing to provide for her child’s welfare, that would be an argument for going. If you believed that boycotting her wedding would irreparably damage your relationship with her – not to mention your relationship with your wife – that would be another argument for going. Indeed, if you have reason to believe that you will be cut out of your grandchild’s life, or out of the position to be a positive influence in your grandchild’s life, that also would be an argument for going. If, on the other hand, you believed that your daughter were only interested in pursuing her own agenda regardless of all that you had tried to teach her about personal chastity and the sanctity of marriage, that would be an argument for not going.
In the end, only you can decide what to do according to what your conscience tells you that you must do. Please remember that your wife must also do the same thing and may come to a different conclusion than you. If you each accept each other’s decision and respect each other’s consciences, that may help to avoid damage to your marital relationship.
Should I Attend? by Catholics United for the Faith