Can a baptized Catholic's marriage outside the Church be valid?

Our godson, a confirmed Catholic, began attending a Mennonite church with his girlfriend when he was 17. Evidently he made a public profession of faith in that church when he was 18. He no longer attends Mass. Next month he (now 20) is planning to marry his girlfriend in the Mennonite church.

Can a baptized Catholic’s marriage outside the Church be valid? What are the duties of Catholics (godparents, grandparents, aunts and uncles, friends) regarding the attendance of such weddings, and what is the charitable action upon receiving notice of such a tragic event?

As your godson has formally left the Catholic Church through his membership in another church, he is no longer formally bound to marry according to Catholic marital law. Assuming that there are no obvious impediments to the marriage (e.g., previous marriage, close blood relationship), the Church would presume his marriage to his fiancee to be valid. If she is a baptized Christian, the marriage would be presumed sacramental.

Recommended reading:

Should I Attend? by Catholics United for the Faith

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