Is this wedding going to be sacramentally valid?

Hello,

Two of my friends have decided to get married, and I am not sure if their marriage will be sacramentally valid. They want to have a purely secular ceremony, meaning that they will be married by their town mayor. My understanding is that such a type of ceremony is only sacramentally valid if they are not Catholic.
Both people were raised in non-Catholic families, and were never educated in the faith properly and have only been to a handful of masses in their life. The problem is that they are both baptized (one by the Church, and one by her grandfather).
I am asking because they are close friends and have asked me to be a witness. I will refuse if there is a doubt about the marriage’s validity. Can you please give me your educated opinion on this?

    Many thanks,

    Robert_Caritas

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