Firstly: 

The questioner has done very well by seeking to advise his friend in the light of Islamic teachings. This is an example of perfect love and Islamic brotherhood. The Prophet (peace and blessings of Allaah be upon him) said: “Religion is sincerity (or sincere advice).” (Narrated by Imam Muslim). If such good care was widespread among the Muslims their situation would be far better than what we see now, especially in countries where Islam and its followers are strangers. The Muslim is in great need of his Muslim brother to give him advice and to love for his brother what he loves for himself. Perhaps a kind word or sincere advice that a man gives and pays little attention to may earn him the pleasure of Allaah, which will save him from eternal misery and doom. 

If your friend really wants to know the truth, he has to be honest with himself and free himself from whims and desires, especially in such situations. 

In the question itself there is the admission that his love for that Christian girl is a sin. How can it be acceptable for a Muslim man to love a woman in this way, when she is not his mahram and not his wife, and it may be said to him, this woman represents the honour of another man. Just as you would not accept for a non-mahram man to love your daughter or your sister when he is not her husband, then others would not accept this from you. 

In such cases of love, the only thing that is permissible is marriage based on the sound principles of sharee’ah, not just love which the questioner himself accepts is a sin. When our pure monotheistic religion set out regulations governing the relationships between men and women, it set out a proper way for these relationships to be conducted. So marriage is the beginning of these relationships not the result thereof. Marriage is not the result of love which is a sin. Moreover, this system of relationships is in the interests of the Muslim society, and its stability and soundness, because going against the teachings of Islam leads to a great deal of corruption and mischief, such as that mentioned in the question which causes grief to the first wife. 

One mistake leads to another, and another, and so on. This young man – may Allaah bless him in this world and in the Hereafter – wants to correct this sin by forsaking his first wife, but this is another sin which is no less serious than the first one; a sin cannot be corrected by means of another sin. 

When Islam permits plural marriage, it stipulates the condition of fair and just treatment for all the wives, so it obliges the husband to treat all his wives fairly. Allaah says (interpretation of the meaning): 

“but if you fear that you shall not be able to deal justly (with them), then only one or (slaves) that your right hands possess”

[al-Nisa’ 4:3]

Undoubtedly it is not just or fair to forsake the first wife in order to marry the second, even if we assume that the intention in this case is good, which is to make the Christian woman embrace Islam, for the Islam of the second wife does not require that the first wife be mistreated. The end does not justify the means. Moreover, there are many ways and means of calling the Christian woman to Islam, all of which may be used. 

How many people have been called to Islam without that involving wrongdoing or committing of sin. 

Based on the above, there can be no doubt that his first wife will get angry; if he were in the same position he would get very angry too. 

Read with me the following hadeeth: “Whoever has two wives and is more inclined towards one of them will come on the Day of Resurrection with half of his body leaning.” (Narrated by al-Tirmidhi, 1141; al-Nasaa’i, 3942; Abu Dawood, 2133; it is saheeh according to Irwa’ al-Ghaleel, 7/80, no. 2017) 

Would you like to come on the Day of Resurrection with half of your body leaning? Undoubtedly you would not like that. 

All of the above applies if he wants to forsake his first wife for a limited period of time. However, if he wants to forsake her permanently, then it is even worse. I do not know which is meant in the question. 

Let us assume that you marry this woman after she becomes Muslim, then you come to know of another woman who, if you marry her as a third wife, that may be the reason for her becoming Muslim, but you do not know her or love her. Will you forsake the second wife so that this one will embrace Islam even though you do not love her? Answer yourself honestly. 

Finally, I do not advise you to marry this Christian woman in such circumstances where the matter is based on a previous sin, which is loving her unlawfully. Yes, you are to be commended for your eagerness to guide someone else to Islam. You could give her some Islamic books, publications and tapes, and tell her about Islamic centers and female Muslim daa’iyahs, or try to get her married to someone who is committed to Islam and who could help her to remain steadfast, but without wronging a previous wife, etc. 

Secondly, it is obligatory to treat wives fairly with regard to spending nights with them and spending on them. It is not permissible to forsake the first wife unlawfully, even if that is with a noble intention. 

Thirdly: The Muslim who is keen to keep his religious commitment sound will keep away from love that is not based on a sound foundation, even if that has a strong effect on the heart at first. Allaah says (interpretation of the meaning): 

“As for those who strive hard in Us (Our Cause), We will surely guide them to Our paths (i.e. Allaah’s religion — Islamic Monotheism).”

[al-‘Ankaboot 29:69]

And the Prophet (peace and blessings of Allaah be upon him) said: “There is nothing that you give up for the sake of Allaah, but Allaah will compensate you with something that is better for you than it.” (Narrated by Imam Ahmad, 20222; classed as saheeh by al-Albaani, al-Silsilat al-Da’eefah, 5)

 You should strive to attain the mercy of Allaah and Paradise by being kind towards the mother of your children and pleasing her as you have angered her, and trying to make her happy after you have made her sad, after she has lived with you for more than seven years.

 May Allaah help you to do all that is good.