Why does a member of the church go to hell if he commits suicide?

Answer:

I'm sorry to hear that you lost a friend. Such events leave everyone with many questions.

Suicide is a form of murder. Instead of killing another person, the person committing suicide is killing himself. Murder is a sin. See Why is Suicide a Sin?

Sins can be forgiven, but it requires that the sinner repent of his sin. This cannot be done when you are no longer living. Of course, no one knows all the details. That is why God judges us, so I can't make a blanket statement, but see: Can suicide be forgiven?

Just because a person is a Christian, it doesn't mean he has immunity from sin. Christians work against sin, but they do sometimes fall prey to it. See How can a Christian sin if we are to be holy like God? Nor do Christians get special treatment. God is an impartial judge (Romans 2:11). "For it is time for judgment to begin with the household of God; and if it begins with us first, what will be the outcome for those who do not obey the gospel of God?" (I Peter 4:17).

We can pray that God finds reasons to extend mercy to your friend, but we must remember that God must also be just. "For we must all appear before the judgment seat of Christ, so that each one may be recompensed for his deeds in the body, according to what he has done, whether good or bad. Therefore, knowing the fear of the Lord, we persuade men ..." (II Corinthians 5:10-11).