You are right; your friend should accept more responsibility for his actions. While Satan does tempt us and tries to lead us astray, the Bible makes it clear that we alone are responsible for our sins.
In reality, however, we don't like to admit this. Instead we blame others for the wrongs we do our parents, our friends, the devil, even God. When Adam and Eve disobeyed God and ate the forbidden fruit, Eve immediately blamed the devil for tempting her (see Genesis 3:13). Adam blamed Eve and then blamed God for giving her to him as his wife: "The woman you put here with me she gave me some fruit from the tree, and I ate it" (Genesis 3:12).
But we alone are responsible for our sins, and we alone are responsible for the pain and heartache they cause us. This is why we need Christ, for only He can change our hearts and give us the power to do what is right. The Bible says, "I can do everything through him who gives me strength" (Philippians 4:13).
Pray for your friend, and urge him to face his own need of Christ. He needs Christ's forgiveness, and he also needs the moral and spiritual strength to do what is right. Satan has convinced him that the path he is on will bring him happiness but Satan's promises are always lies. He needs Christ, for only He can give him the joy and peace he seeks.