There is only one sin that God will not forgive and that is refusing to except Jesus Christ as your Lord and Savior. While suicide is considered a sinful act, if your friend excepted Jesus "before" he died he will be in heaven with Him. This is the ONLY way to heaven.
And to your statement "we have never died and don't know for sure." Yes, we do know. The Bible tells us in John 3:16 "For God so loved the world that He gave His one and only Son, that whoever believes in Him shall not perish but have everlasting life."