Attending a civil wedding

peace,
I was invited to a colleague's wedding. He and his fiancée do not want to get married through the rabbinate by a rabbi but by a "wedding officiant." On principle, they are not interested in undergoing bridal training, baptism, etc. Not out of hatred, but because they want to do it the way they see fit and not perform any "religious" duties that they do not identify with at all. The truth is, from their perspective, I quite understand them.
The question is: Can I/should I attend the wedding or is it better not to? I can easily come up with a convincing excuse so that there is no fear of harming his dignity.
Thank you very much,