It depends. If one or both of the spouses is/are Catholic then the marriage would be invalid unless they received permission from the bishop to have the wedding in the Protestant church. If one of the spouses had been previously married, then the marriage would be putatively invalid. In these cases, it’s best not to attend. If both parties are non-catholic and never married before, then it would be a valid marriage and you should attend gladfully and prayerfully.