Extra marital affair with an unmarried women and had a child

Hi,

One of my Brother's Friend had the below issues and I need suggestions from the experts on this. (Referred as "Married Man", henceforth)

He is a married man with a 11 year old Child. Since 4-5 years, he has been in an extra marital affair with an unmarried girl. The girl knows that he is a married person but still engaged with him. During this period, she got pregnant and girl wanted to keep the baby and a baby girl is born. Now, the baby is 3.5 years old and all the living expenses from starting were borne by the "Married Man" and that girl is also happy with him. The "Married Man" is also doing all his responsibility of child as well as mother. Now, since 6 months she has been dating some other boy. That boy too knows that she had a baby. Now she is deciding to marry that boy. (May be they both planned to grab more money from the "Married Man", since he is a established person).

Now, she is telling the "Married Man" to be away from her and also asking to give a certain amount of money on monthly basis, else she will come with Women Cell to his family and do legal action. Even, the "Married Man" is providing all the monthly recurring expenses, but she is asking more as per her wish and blackmailing him. The Girl child is also admitted to School, and all the expenses were done by his father. The "Married Man" is closely attached to his daughter (the 3.5 yrs Girl child), hence he got mental distress as well.

Now, my queries are as below;

1.) What can the "Married Man" do to get rid of this Unmarried woman? The "Married Man" is providing financial support now, but in future may be she will ask for some big Amount and again blackmail him.
2.) What is the right of "Married Man" on the born Child? 
3.) What that Girl can do legally in this case?
4.) Does the "Married Man" can get custody of the Child (if he wants)?
5.) As per the LAW, what is the legal responsibility of the "Married Man" towards her and the Girl child?
6.) What should the "Married Man" do, so that he will be legally safe in future?

Please advise all the Legal Consequences in this case.