Q. I have read your article, Virginity, the Priesthood and the Polygamous Patriarch. How would you help guide a woman overcome her past mistakes before she understood the gospel? What advice can you give in this regard?
One of the problems in tackling such issues is that everyone has to be treated somewhat differently, especially when it comes to the details, and so one can never give specific counsel but only in the most general sense.
If I am reading you correctly between the lines, you are interested in someone who is not a believer but has led a promiscuous sexual life? If this is the case, I am sure you will know that considerable time MAY be required until such a person is ready for marriage, let alone polygamy. True and not mimed salvation is absolutely essential for proper cleansing or you will inherit a bundle of mischief that may ruin you. Following salvation, which is only Step #1 in the process of sanctification that would ultimately bring about marriage, for the next you will need proper spiritual DELIVERANCE from demonic strongholds. If she has fornicated and committed adultery, this will first require a complete and free confession of all previous liasons (itself difficult, but vital) and genuine repentance of such. Without the latter, deliverance is not possible, and polygamous marriage inconceivable. You must also have the self-discipline to insist on a year's total and complete celibacy from the time of the previous sexual liason (from the time you met mightn't be a bad idea either) before marriage is contemplated. If she is not willing to do this then I would suggest she is not true candidate material for either Christian/Messianic monogamous or polygamous marriage. (Proper screening for sexually-transmittable diseases is, of course, also essential).
As to how to 'do' all of this, only the Holy Spirit can guide you, with the help of a counsellor if both parties are willing. If at some time your female friend would like a woman counsellor, just get in contact with us again.
Finally, this is something she must want of her own free will. Any kind of pressure will not lead to true salvation or deliverance, much less a happy marriage. Much intercessory prayer on your part will likely be the order of the day for some time.
You are unlikely to be able to lead an unsaved person to a proper understanding of the importance of sexual deliverance and purity before salvation. You may convince her of the medical benefits of such, and even the desirability of transparency in any future relationship that such would bring, but until there is that resurrection power resident within her, deliverance from the past will be impossible.
This is my counsel if you are seeking for a quality marriage built on a secure and happy foundation. Anything else will just bring you trouble and sorrow.
 How Important is Sexual Purity in Plural Marriage? (plus extensive reading list)