Because he is allowed to only marry a Muslim, a Jew or a Christian and
this woman who doesn’t pray is neither one of the three any more.

my son is asking ,why is a muslim man adviced to seperate from a muslim wife,who dosent pray,when its allowed for him to marry a jew or a christian? We are confused about this.

Invalid Marriages | November 6, 2016 3:31 pm
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If a very close relative gets married with a Christian boy where parents do all the arrangements ,what should be our behaviour towards girl and parents.
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I have a (Christian) colleague who reverted to Islam. However, his wife was already a Muslim when they married. In this case, do they have to repeat their Nikah? They have kids as well.