I'm going to answer to Tirzah's comments to my previous post here. I want to make it clear that those were Fr. Ripperger's opinions and while I totally agree with some of them, I think there's a lot of room for discussion for some of the other issues presented.
In the case of the "marriage" outside of the Catholic or Orthodox church, it is reasonable to consider the union between those two people, just that, a "union". A common law marriage is obviously not sacramental. Two people living together as husband and wife outside of a sacramental marriage, are committing the sin of fornication. By taking part in a non sacramental wedding you send the message that you, as a Catholic, agree with the licitness of the union. I know that sometimes it can be very hard and you could be in a very delicate situation, risking to offend and maybe even loose a friend, but at the same time, your decision to not take part in this, might send a message to this friend or relative, it might make them understand the gravity of living their life outside of the Church. I do think though that not accepting the couple in your house would definitely be too much, but I think Father nuanced this a little. As I said I don't remember everything he said, exactly the way he said it.
I would definitely be friends with someone who lives in a non sacramental marriage, but, I wouldn't do anything to send the message that I agree with their lifestyle. I agree with you Tirzah, we have to be charitable towards those who don't have the same religious background as us, but I think that, at the same time, we have to work hard on bringing them back to the Truth. That would be real Love, Friendship and Charity.
PS. I reread your comment, Tirzah and I have to add something. I think that in your cousin's case (was it Erzsi?) it is a sacramental marriage for her but not for him, because he is not baptized. I'm not a theologian and I'm not trying to be a "know-it-all" but I remember that I have discussed this very scenario (Catholic marrying a Muslim) with my aunt a couple of years ago and because we disagreed on our opinions, I called my uncle in Boston, who is a Catholic priest and I asked him about this. He said that if the two marry in the Catholic church and promise to raise their children in the Catholic faith, the marriage is sacramental for the Catholic person.
I think you did well by deciding to attend their wedding and be her bridesmaid. There's nothing wrong with that. She is not living in sin. Hopefully she will pray a lot for her husband so that he will eventually convert to Catholicism. By the way that picture of the two of them is very cute. What a beautiful couple!