I have a friend who is encouraging her brother to "marry" his "partner." It's really weighing on me spiritually. How do I convince her that this is not the right path?
You may not be able to convince your friend, but God certainly can. He can do that through his word. What you want to do, then, is put your friend in touch with God’s word and let the Holy Spirit take over.
What you want your friend to understand first is that God’s word is truthful in whatever it says (John 17:17). Again, that is a conviction of the heart that only God can bring about. You can pray that God will convince your friend of the truth as she comes into contact with his word.
As far as resources are concerned, you might be interested in reading our church body’s statement on homosexuality. This link will take you to that statement. It contains Bible passages that could be shared with your friend.
Additionally, this link will take you to an article titled “Defending Traditional Marriage Is an Act of Love.” The article is from Christian Life Resources, an agency within WELS. You will find other resources on this topic at their web site as well. God bless your witnessing efforts!