I am convinced that the vast majority of these people are where they are because they really want to serve God with all their hearts. These people are not God's enemies nor are they ours. They are victims of false teachers. Encourage Discussion on Life issues to get some insight on how your friend perceives and understand things. Get him to talk about the things which concern him and lead him into a discussion about what his group means to him.
People are in cults for many different reasons.
In no way let him come to the understanding that you only care for him as long as you might convert him. In Some Way Give Your Own Testimony. A good place to begin is to tell specifically how God has answered prayer in your life. Avoid testimonies that talk about the peace and joy you have in the Lord. Remember, those who follow Eastern religions and any number of the new cults will give just as fervent a testimony.