NeedofHelp, I believe the main thing you can do for your friend is your prayer. Pray always for your friend, don’t rely on your strength, wisdom, and understanding, but let the Lord deal with the situation. Your friend needs God's grace more than anything that you can give her. Therefore, never stop praying.
In my opinion, the LC is a cult and its members are cult victims. Usually, cult members refuse to listen to any independent sources of information that criticize the cult and its teaching. The information is considered invalid because it comes from an outsider. And you are an outsider, while the LC members are “the chosen ones”. The LC members believe that Christianity is fallen and it’s only them who know “the high-peak truth”. That doesn’t mean that you can’t do anything or you have to join the cult to help your friend. I just want to say that it can be hard for you to persuade your friend to leave the LC. She doesn't trust your opinion because you are not an authority for her in this matter. You may tell her the truth about WL and his son, but she won’t believe the facts. You may tell her that the LC substitutes the word of God (the Bible) for the word of man (WL and his books), but it will be a miracle if she notices the substitution.
You can try to change your approach. Don’t tell your friend everything you know about the LC. Do the opposite. Start asking questions. Don’t be pushy. Just ask a question from time to time: How often do you read the Bible? Why do you read the Morning Revival and Crystallization Study, not the Bible? What was the last message about? How does this message relate to Christ’s message? Can this “high-peak truth” be higher than Christ’s words: 'Love the Lord your God with all your heart and with all your soul and with all your strength and with all your mind'; and, 'Love your neighbor as yourself'? Can “the high-peak truth” help us enter the kingdom of God if we don’t love our neighbors (who are everyone around us and not saints only)? What makes you think that the LRC is the only genuine Christian church? Why do you rely on one man’s opinion about the Bible? What makes this man to be an authority? Could he make mistakes? If he could make small mistakes, could he make crucial mistakes as well? What made WL change his opinion and vision from time to time? If he is infallible, doesn't he remind you a pope? Can I share with you another interpretation of the verse? Why do saints leave the church? Are all these saints ambitious, proud, and arrogant? If she knows the truth about the hidden history of the LC, ask her if a bad tree can bear good fruit? Can we be saved, eating fruit of a bad tree? What did Jesus say about false teachers? What are their fruits? (Matthew 7:15-20) What are their character qualities? Greed, pride, and rebellion? (Jude 1:11) What kind of man was WL in his private and church life? What did he say about Christianity and other Christian churches? What are characteristics of denominations? Can the LC be called denominational church of Witness Lee? Are Christ's message and WL's message the same? What about "the high-peak truth?" Does WL go further than traditional Christian churches? What is this upgrade for? If the gospel of Christ and His words remain the same, then why does the Bible need an upgrade and update (or recovery)? Where does Jesus mention WL's "high-peak truth"? How many Bible verses support "the high-peak truth" in the Morning Revival? Why does this "high-peak truth" take priority over Jesus's words? Does this "high-peak truth" help you to love God and your neighbor more? Where is Christ in the high-peak truth of Witness Lee? Can you say that you can see Christ in WL's books more than you see Him in the Gospels? If not, then why do you start your day with the word of man (Morning Revival) and not with the word of God (the Bible)? What if WL's doctrines don't lead you to Christ but somewhere else? What about the Apostle Paul who says in Galatians, “Evidently some people are throwing you into confusion and are trying to pervert the gospel of Christ... If anybody is preaching to you a gospel other than what you accepted, let him be eternally condemned!”
Pray to the Lord to grant you wisdom and ask right questions in the right time. Plant your seeds step by step. Help your friend to question her beliefs. But again... Don’t behave like an enemy to her. Be her friend. Always.
You may also try to email her some testimonies like this:
http://jubileeresources.org/?page_id=102 (Thanks to Aron!) Ask your friend to explain some points; ask what she thinks about the testimonies. But don’t bomb her email account with testimonies day and night.
Read more about cults to find out similar characteristics between the LC and other cults. If your friend takes you to an LC meeting, pray to the Lord, before, during, and after the meeting. I don’t recommend to attend the LC meetings, but if you happen to be there, you must be warned and fully alert of the cultish system so that not to be deceived by the external state of joy, goodwill, and happiness.
http://www.leaderu.com/common/cults.html
http://www.the-gospel.org/stdy_hrmnt....php?pass=prnt
http://www.orange-papers.org/orange-cult_q8.html
Last but not least, start changing yourself. I’m sure you are a good Christian, but there is always a way for perfection. “Be perfect, therefore, as your heavenly Father is perfect”. (Matthew 5:48) Read and study the Bible, fulfill commandments, pray more, developing your personal relationship with Christ. Glorify His name with your thoughts, words, and actions. Fight your passions. Don’t gossip, don’t discuss people, and don’t waste your time with idle talk. If you speak, speak with love, humility, and kindness. That’s difficult for everyone of us. I’m not an exception. But if we want to help our friends and relatives to leave the darkness, we must become a torch for them. To do that, we must let the light of Christ to shine through us. That doesn’t happen overnight. You will need to make efforts to live in communion with God. But if you live in strong connection with the Lord and bear the fruit of the Holy Spirit, then it will be easier for you to show the right way to your friend. You will not be a torch holder but a part of the light. If your friend sees Christ in you and your life more than in her church life, she may wonder and start questioning. And one day she may say to herself, “How come my friend has no idea about the LC high-peak truth but s/he is so full of Christ? Is there something wrong with my spiritual life? I had better spend more time with my friend. Maybe s/he can show me a better way to Christ”.
May the Lord’s love, kindness, and humility dwell in you. God bless.