The short answer to this one is: Truthfully. How would your church respond to this couple? One of the things we promote at my church is radical hospitality with Biblical clarity. Everyone is welcome to come, hear the Gospel and build relationships. There is no litmus test for love and friendship. I’ve been blessed to see our church receive people who, there’s no other way to put this, “stand out” from the average attender. They are welcomed and treated respectfully. We’ve had same-sex couples visit, interact with us and be treated with love and respect. If your congregation has this same, outreach-oriented heartbeat, then feel free to let your friends know they’ll be welcomed.
At the same time, a church that embraces Biblical teaching on this topic will eventually bring this couple into contact with the message that God has a different plan for human sexuality than the one they are pursuing. Sometimes, that happens right away, because they ask. Sometimes, it comes a little further down the road. But it will come. At that point, your friends will be facing the question that Jesus asks all of us, “Are you willing to lay down your life and take up the Cross?”