Mother name in the passport

[faroquex]

Hello
One of my friend recently got student visa and now he is planning to apply for his family also. He has two kids. My friend is divorced and living with new wife. My question is that when he will apply for family visa what should he writes his children mother name in the application form as well as in the passport application. Please children have birth certificate with original mother name. Any suggestions would be highly appreciated.

Thanks.