She is dating a Christian man and he has promised to become Muslim and marry her
First of all, allow us to remind you of something that you neglected in your question, and it seems that you are also neglecting it in your life, whilst you are continuing with this behaviour and are far away from a good environment and a society that reminds you of Allah, may He be glorified and exalted. This thing is the fact that your dating your boyfriend is haram, and it is haram to be in contact with a man in a love relationship outside of a legitimate marriage, regardless of whether that man is a Muslim, a Christian or anything else. Undoubtedly the sin and shame are greater if the man is a disbeliever, because in the case of a Muslim there is the hope that you may get married to him, but in the case of a disbeliever, that is nothing but wishes and the deceit of the Shaytaan who makes these things seem good.
This kind of relationship will either involve haram acts – Allah forbid – or it will be a path that leads to them. The Shaytaan lies in wait for man and seeks to lead him astray, and the human soul is inclined to follow whims and desires. No man is ever alone with a woman but the Shaytaan is the third one present. Allah, may He be glorified and exalted, says (interpretation of the meaning):
“O you who believe! Follow not the footsteps of Shaitan (Satan). And whosoever follows the footsteps of Shaitan (Satan), then, verily he commands Al-Fahsha (i.e. to commit indecency (illegal sexual intercourse, etc.)), and Al-Munkar (disbelief and polytheism (i.e. to do evil and wicked deeds; to speak or to do what is forbidden in Islam, etc.)). And had it not been for the Grace of Allah and His Mercy on you, not one of you would ever have been pure from sins. But Allah purifies (guides to Islam) whom He wills, and Allah is All-Hearer, All-Knower”
You have asked for our opinion, not on the basis of our personal opinions, but on the basis of our understanding of what Allah loves and is pleased with, and what is taught by the religion of Islam which we all follow, praise be to Allah, and which we believe is the way to happiness in this world and the Hereafter.
We will give you sincere advice, as you would expect, by Allah’s leave. We say to you: marrying a Christian is definitely forbidden in our religion, and it is one of the most reprehensible deed that a Muslim women could do. That is not marriage in any way whatsoever; rather it is pure sin and a great evil, as Allah, may He be glorified and exalted, says (interpretation of the meaning):
“then if you ascertain that they are true believers, send them not back to the disbelievers, they are not lawful (wives) for the disbelievers nor are the disbelievers lawful (husbands) for them”
The prohibition is not limited to a case where you fear that he may make you enter his religion, rather even if he promises you that he will become Muslim, it is not permissible to rely on his promise; rather it is essential that he actually does enter Islam out of a genuine wish to follow this religion and believing that it is sound, and it is essential that he becomes a sincere Muslim – all of that should come before doing the marriage contract.
Without that, we do not think there is any confusion regarding your question; rather the matter is quite clear. What you must do is repent from what has happened in the past of this haraam relationship. Allah, may He be glorified and exalted, loves those who repent and He loves those who purify themselves. If the Muslim is faced with a choice between what is halaal and what is haraam, and he chooses what is haraam, then he has no one to blame but himself, and he will see the consequences of that in this world before the Hereafter. But if he chooses what is halaal, and seeks reward for his patience and perseverance with Allah, may He be glorified, Allah will reward him, by His leave.
Being guided and helped (to make the right choice) is in the hand of Allah, may He be glorified. All that the individual is required to do is take appropriate measures and weigh things up by examining them rationally. It is essential to ask about the situation of that Muslim who has proposed marriage to you, and what he has of religious commitment and good characteristics that make him suitable to be able to treat all his wives equitably. What you mentioned, about him only having married his first wife recently, is not an impediment to marrying him if he is suitable for marriage, and it is not an excuse. Moreover, you have no excuse for giving precedence to the haraam relationship over him, or for waiting for a long time with no guarantee. Plural marriage in Islam does not require waiting for a specific length of time between one wife and another, or a specific age; rather the matter depends on the wishes of each partner and their compatibility.
Pray istikhaarah, asking Allah, may He be exalted, for guidance as to whether you should agree to this marriage, and ask Him to make it easy if it will lead to your happiness; otherwise it is better that it be diverted from you by His protection.
As for the first relationship with a non-Muslim, it is not permissible to pray istikhaarah concerning it, because istikhaarah can only be done with regard to things that are permissible, not things that are haraam. So you must immediately cut off all ties with him and get rid of everything that may remind you of him or pull you back into that relationship.
And Allah knows best.