You need not now down to pressure tactics 

2) if girl files rape  case apply for and obtain anticipatory bail from session court 

3) take the plea it was consensual affair with no promise of marriage 

This is a blackmail, harrasment and coercion hence your friend should not rush to marry her out of fear.

Consensual sex between two adults is not rape 

Her acts amount to criminal intimidation.

The pregnancy does not create right to marriage.

Your friend can collect all documentary evidences to prove the false allegations.

He should not meet her or her family alone. No emotional arguments. Everything should be documented.

Before she lodges any complaint you should lodge a criminal complaint against her for the acts of threats, criminal intimidation and coercion along with false allegations made by her demanding illegal demands Even before any FIR is filed, if threats continue. Courts routinely grant AB in such coercion cases, especially with evidence. Hence you may consult an advocate and prepare for filingp a AB.

If you marry her then the annulment is not possible for any reason specifically for the reasons you stated 

If yiu are forced to marry on grounds of coercion file for anullment of marriage within period if one year of marriage 

First and foremost, he should not agree to marriage under threat or coercion. A marriage entered into because of fear of false criminal cases, suicide threats, or police pressure is not a genuine consented marriage and often leads to further legal complications rather than solving the problem.

Consensual sexual relations between adults do not amount to rape merely because the woman becomes pregnant or later demands marriage. Courts have repeatedly held that rape is made out only if consent was obtained by a false promise of marriage from the very beginning, made with no intention to marry. From what you have stated, the relationship was clearly casual, there was no promise to marry, and there is even a call recording supporting this. Threatening to file a rape case to force marriage can itself amount to criminal intimidation and extortion. Your friend should immediately preserve all evidence—call recordings, chats, messages, and any proof showing consent and absence of a marriage promise.

He should consult a criminal lawyer locally and be prepared to seek anticipatory bail if an FIR is lodged. He can also give a preventive written complaint to the Superintendent of Police stating the threats, blackmail, and false implication risk, enclosing evidence. This often helps create a record and discourages misuse of law.

Now, regarding your specific question about annulment or divorce if marriage happens:

If he marries her under pressure and later wants to exit the marriage, the law becomes far more complicated.

Annulment is possible only on limited grounds such as fraud, coercion, or lack of free consent, but proving coercion is very difficult in practice once the marriage is solemnised and consummated. Courts are cautious in granting annulments, especially where pregnancy or a child is involved.

If she gets an abortion after marriage, that does not automatically give him a ground for annulment or divorce. Termination of pregnancy by itself is not a matrimonial offence unless it is done without consent and accompanied by cruelty, which again requires strong proof.

If she gives birth to the child after marriage, the situation becomes even more complex. He will be presumed to be the legal father, with obligations of maintenance, child support, and inheritance, irrespective of how the marriage began. Divorce may still be possible, but it will likely involve:

• maintenance claims by the wife,
• child maintenance,
• custody and visitation disputes, and
• prolonged litigation.

In short, marriage does not neutralise the threat—it increases legal exposure.

The most legally sound course right now is:

• Do not marry under pressure.
• Secure and preserve all evidence.
• Consult a criminal lawyer immediately.
• Be prepared with anticipatory bail if needed.
• Consider a pre-emptive complaint against blackmail and threats.

Once a marriage takes place, the legal leverage shifts heavily, and remedies like annulment or quick divorce are neither easy nor guaranteed—especially if a child is born.

Your friend should treat this as a legal emergency and act proactively, not reactively.

He can fake anticipatory bail and not marry her. She can’t press false rape charges in consensual sex