Your friend should take the plea that it was consensual sex with no promise of marriage 

2) in event lady files rape case apply for and obtain Anticipatory bail from sessions court 

Dear Client,

Your friend needs to get out of the same. He can file a police complaint against her for blackmailing and threatening. If she files false FIR he can take anticipatory bail to secure himself and avoid arrest. 

See she cannot file any complaint against the person because she is in relationship knowing he is married ,and there is no false promise of marriage in case the lady intimidate him and threaten then he can file a police complaint against the lady.

you should lodge a police complaint to the nearest police station or alternatively issue a lawyer's notice to her. 

live in relationship and consensual sex between to adults is not crime.

The right advice would be that he should tell all this to his wife and apologise for his act of adultery a d then they can together take on this widow. She can't do anything.

But I dont know about his relationship with his wife. Therefore I would advise to stay away from her and inform the women's commission and the local police station about it in case.

Regards 

if she makes any complaint in the police against your friend, he will be in grave crisis,

he may be charged with rape on the false promise of marriage, sexual harassment, beatings, threats, blackmail, etc

tell your friend to settle the matter amicably by convincing her about his married life which I think is very tough,

alternatively,

your friend can make a police complaint against her, taking his family especially wife in trust, for blackmailing, extortion of money, mental harassment, etc.

You cannot be forced to stay with lady 

2) no need to bow down to pressure tactics 

It's better you keep distance from that lady.  She can't force.  She can put the person in difficulty who will be with her. You can take my detailed telephonic consultation from kaanoon if you required any complex information regarding the same anytime. 

No she cannot force or charge you. See for that she need proof of marriage

No she can't force you to live with her. If the relationship was consensual the she can't charge you.

no one can force anybody to live forcefully with him/her,

"if the physical relation has been made then can she charged me for that."
are u also involved in the matter? 

anyway, if physical relations had been established, the person can be charged with RAPE.

marriage is illegal, if performed, as your friend is already married and had not taken Divorce from his wife

your friend should deny all allegations and issue a legal notice to her. 

your friend can also file civil defamation case against her. 

Your friend can ignore her threats because she cannot do any harm legally to him.

She may be interested in continuing her relationship with him hence out of the disappointment she may pose such threats, she may be asked to proceed because her complaint is not maintainable anywhere in law.

She may give any strong reason to threaten you, do not budge to any kind of pressure from her side.

She cannot do any harm to you in any manner except to spread their illicit affairs everywhere including your friend's wife and others.

1. As per the information mentioned in the present query, makes it clear that the lady had an idea about your friend’s married life.
2. It means that if she would have been intimated with your friend then she were with her own free will and consent.
3. Now, legally she can’t file case of cheating or rape on the pretext of marriage as she knew about his married life.
4. She can only succeed if your friend would have committed her that he would give divorce to him first wife and then marry her, which almost seems impossible as of now.
5. If she is saying that she is married then also that marriage is void as per law as the first marriage is in existence.

Dear Client 

Your friend landed yourself in a big trouble of extra marital affair and as the lady is blackmailing him to live with her.

The best thing your friend can do in current stituation is that make a call to her and record call in which he should ask that the relation between them was consensual and take her agreement on that so that he can use recording in his defence when the case of sexual harassment or rape is filed by her.

And he should file for anticipatory bail to avoid arrest in case she lodge complaint with police.

And also if the blackmailing continues he should lodge a police complaint against the lady for blackmailing for levelling false cases on your friend.