This is a matter of prudential judgment. The standard response would be to not attend the wedding, because it will be an invalid marriage, and one should not celebrate or make it appear as though one condones an invalid marriage. However, there is an argument that some will make, depending on the situation, that if not attending will create a permanent rift in the relationship, then it might be better to attend, so that through an ongoing relationship one can give witness to Christ and the Truth over the longer term. If you have children of an impressionable age, it might be more important to keep them from attending, so that they are not confused.
I think you have to ask yourself questions like:
–are the members of the couple reasonable enough that they can respect your commitment to your faith? If you tell them, “I have to be true to my conscience” will they respect that answer?
–if we don’t attend the wedding, will that cause a permanent end to our relationship, or will it damage it so badly that we will never be able to have a positive effect on them?
–Is the couple so completely removed from their faith, and is their understanding of it so stunted, that they won’t be able to understand my reasoning? And do I need to witness to the faith in more basic ways instead?
–Are there other people who will believe that this sort of wedding is ok if we attend it? Are we likely to cause scandal to other people?
–Will I be able to attend the reception, and have the type of conversation that is usually had at a wedding (like, “wasn’t that a beautiful ceremony?” “aren’t they the perfect couple?”), or will I be able to hold back from being dishonest without creating a stir?
Certainly talk to your priest about it. He may be able to give you advice more appropriate to your specific circumstance.