Sometimes you have to have one of those conversations. You know it's the right thing to do. You love the person and can no longer sit by watching them self-destruct. No way around it - there are knots in your stomach that aren't going anywhere. You're probably going to lose some sleep. Possibly lots of sleep. It won't turn out to be the Tuesday or Friday that you were planning on. (Does it ever?) But you know in your heart - because you live by a standard that is never moved or shaken - that this conversation has to take place. And it's probably going to hurt.
[I think it's important to clarify that what we're talking about here is known sin - a friend or family member who claims to be a "Christian" clearly rebelling against the standard of God. This is not a license to confront anyone and everyone who falls short of our own expectations. 1 Corinthians 5:12 and 1 Peter 4:17 tell us that as believers, we are called to confront sin - in our own lives and in each others.]
You'll begin looking for every loophole, exit strategy, excuse, or way out that you can find. Something or some reason to finally say to yourself, "Wait a minute. This isn't my responsibility! So and so can deal with this." Good luck with that. So and so probably already had the chance to deal with it and blew it. Or possibly didn't care like you do. And that's why you're going to have this conversation.
At some point you'll begin praying for peace. You'll quote Philippians 4:6-7 like my son quotes Nacho Libre - asking God for that "peace that passes all understanding..." Guess what. If by "peace" what you mean is that your stomach will feel better, you'll fall asleep and rest like an angel, and that you'll just begin to have this overall good feeling that, "You know what, this isn't going to be so bad after all. I'm going for it!", then stop waiting. That's not peace. That's a fairytale of some sort that I've never been part of. And peace is not something you can have that's birthed from your emotions or your guts. Peace comes from something unchanging and unmovable. That would be God's Word.
Read Matthew 18:15-20.
Check out Galatians 6:1-3.
(And while you're at it, take a look at Philippians 4:4-9.)
There will be a moment when you cross a threshold and (to put it bluntly) there's no more praying to do. You have to trust that the Holy Spirit's going to be "interceding for you" and is going to give you the words to say. You also have to trust that the Spirit of God is preparing, humbling, and breaking the other person's heart already - preparing them to receive it. If the Lord has been prodding and convicting you that you must confront, then He most certainly will be preparing that person's heart to be confronted. This doesn't mean it won't sting. It also doesn't mean that they won't be defensive.
Is it possible you've prayed enough, and now it's simply time to make that call, look that person in the face, and have that conversation?
Stop waiting for peace. You'll find the peace of God in doing what is right out of love.