Brother,
I hear you, I understand how your heart breaks as I watch my wife throw away our marriage to chase a guy 20 years older than she is.
What future is there in a relationship with a married man who would cheat on his wife.
I'm saying that to let you know I understand the feelings you are feeling right now.
It's apparent that she is hurting inside. She has some sort of pain or unfilled emotional need that she is trying to "medicate" away.
You can't make her quit, but you can set boundaries, such as you won't give her any money, you won't be with her when she is using, etc. However, you can share with her that you do care about her, and want her to experience the best life has to offer.
My prayer for you two is that God will show both you and her His will for your lives. That you both will allow God's Holy Spirit to work in your lives.
I know it seems strange, but sometimes we just have to get out of the way and let God work in her life. It works.
I've been my own worst enemy since my wife moved out 7 months ago. Over the past month, I've really been able to finally let go of my anger and have a more forgiving heart. I've really seen how I contributed to her unhappiness, and how the things I do, push her away.
I have this quiet confidence that she will return to the marriage, but not because of any specific action I do. Rather, I think the changes I have allowed God to make in my life are beginning to bear fruit.
That's the best thing you can do, allow God to work in your life, and to let the joy of your salvation show. I believe if you pray for that as well as praying that God will show her there is a way that does satisfy, she will see the truth.
See Paul's letter to the Colossians. In Colossians 1:3-14
Quote:
Thanksgiving and Prayer
We always thank God, the Father of our Lord Jesus Christ, when we pray for you, because we have heard of your faith in Christ Jesus and of the love you have for all the saints-- the faith and love that spring from the hope that is stored up for you in heaven and that you have already heard about in the word of truth, the gospel that has come to you. All over the world this gospel is bearing fruit and growing, just as it has been doing among you since the day you heard it and understood God's grace in all its truth. You learned it from Epaphras, our dear fellow servant, who is a faithful minister of Christ on our behalf, and who also told us of your love in the Spirit.
For this reason, since the day we heard about you, we have not stopped praying for you and asking God to fill you with the knowledge of his will through all spiritual wisdom and understanding. And we pray this in order that you may live a life worthy of the Lord and may please him in every way: bearing fruit in every good work, growing in the knowledge of God, being strengthened with all power according to his glorious might so that you may have great endurance and patience, and joyfully giving thanks to the Father, who has qualified you to share in the inheritance of the saints in the kingdom of light. For he has rescued us from the dominion of darkness and brought us into the kingdom of the Son he loves, in whom we have redemption, the forgiveness of sins.
Pray for her and yourself as Paul prayed for this church that you both a filled with the knowledge of God. Not facts and figures, but that you both understand how God loves all of us, and wants us to find satisfaction in working for Him.
This life is not about us. God created us to have a relationship with Him first, and then our relationships with family and others here on earth.
I encourage you to read Rick Warren's "The Purpose Driven Life..." it is an excellent text for seeking God's will, His purpose for your life. It's meant to be read over 40 days, a chapter each day with a question to think and meditate on.
I hope this all helps, and I'm lifting you both up in my prayers.
TB 
