No, it isn’t God’s will for him to continue doing this. God doesn’t want us to cloud or confuse our minds in any way, whether with drugs, alcohol or anything else.
When we do things like this, we endanger ourselves morally, spiritually and even physically—and that is never God’s will. We also compromise our witness with others (including some who may be struggling with addiction, or may be tempted to begin using drugs or alcohol in their search for inner peace).
God gave our bodies to us, and He wants us to treat them wisely and use them for His glory. The Bible says, “I urge you, brothers, in view of God’s mercy, to offer your bodies as living sacrifices, holy and pleasing to God—this is your spiritual act of worship” (Romans 12:1) God also gave us our minds, and they too are to be used for God’s glory. Jesus commanded us to “Love the Lord your God … with all your mind” (Mark 12:30). One reason God gave us His Word, the Bible, was “to stimulate you to wholesome thinking” (2 Peter 3:1).
Pray for your friend and encourage him to grow spiritually. Pray not only that he will turn his back on this habit, but that he will put Christ first in his life. God’s will—for him and for every believer—is that we would be “transformed into his likeness with ever-increasing glory” (2 Corinthians 3:18).