Since i am in one and have seen it for what it is. The advice a priest gave me is that the children need both a father and a mother. I sleep on the couch.
It is a very difficult stand to make. More difficult than I ever thought it would be. The strain is amazing. It is a heavy cross when and there is a definite separation the children see. Actually, the idealistic me thought it would be a chance to work on our friendship, but it has not been the case.
The focus on our children's welfare and just taking care of household things are still there. So what should happen by others is just pray for the couple, and be kind to them. If they are not ready to hear about the sin they live in, then your strong views will not help. If you are a genuine friend, one day, the subject may come up and you can help them. Encourage them to go to Adoration, and commit to personal prayer. God will bring them around.
Sometimes people come to an agreement between themselves and begin to work on the situation through the church. Often, people really do not want to offend God on purpose. The entire rest of the world says it is ok to marry, divorce and remarry. So they would thing you are asking too much. Frankly, it is asking more than you know.
Since God loves us all, if we really want to please Him, He will bring us into full knowledge in His most wise ways.