Must you tell the truth about your marital status on a passport application?

There is no realistic circumstances where lying will help her or spouse.

Now, what might be helpful is that the name on the passport doesn’t match her spouses. If that is what your friend is after, she can just continue to use the name she was born with (or make up another name entirely, she’s not stuck with the one she was born with, in some states she can change it entirely simply by starting to use a new nam).

Lying to the government is always lying to the government, and it naturally takes a dim view of it. But saying your last name is Chewbacca is only a lie if you don’t go by Chewbacca in at least some circumstances.

If there’s something else your friend is after, I would suggest consulting an attorney.

The other answers are rather alarmist. Whether the false statement is illegal depends on whether it is material to the passport application, which it likely is not, but in any event this would be a question for the jury at trial.

A false statement is material if it has "a natural tendency to influence, or is capable of influencing, the decision of the decision-making body to which it was addressed."

(From the Justice Department’s Criminal Resource Manual)

The decision to grant a passport does not depend on the marital status of the applicant, so the statement is arguably not material. But the passport authority is likely to take a dim view of that argument. Is she prepared to go to court to present it in her defense in a perjury prosecution?

I haven’t found a case bearing directly on a materiality requirement in 18 USC 1542, because few US citizens if any are prosecuted for lying about marital status on passport applications, but US v. Alferahin shows that there is such a requirement in the very similarly worded 18 USC 1425. Further, it finds that a false statement about marital status is not material to the case at hand. If such a statement is not material to naturalization, it cannot be material to a passport application.

The only evidence I’ve found so far related to prosecutions under 18 USC 1542 are cases in which the defendant is accused of having used a false passport application to obtain a passport under a false identity. Lying about marital status does not approach that level of fraud.

I have asked a question over at Law in the hope of finding some better references to support this answer: Does 18 USC 1542 contain a materiality requirement?

If the Indian spouse is not a legal US resident, it may be possible to get residency on the basis of the marriage to a citizen. Lying on the passport application will make that possibility unlikely—but at great cost to the liar.

However, if the marriage was done with the intent of helping an immigrant get a green card, and she now regrets it, lying on the passport application is one way to get both of them in trouble quickly. Better to own up to the mistake and get the marriage ended. And the false motivation is grounds for doing so.

For what it’s worth, here is text copied from the actual DS-11 (06-2016):

WARNING: False statements made knowingly and willfully in passport applications, including affidavits or other documents submitted to support this application, are punishable by fine and/or imprisonment under U.S. law including the provisions of 18 U.S.C. 1001, 18 U.S.C. 1542, and/or 18 U.S.C. 1621. Alteration or mutilation of a passport issued pursuant to this application is punishable by fine and/or imprisonment under the provisions of 18 U.S.C. 1543. The use of a passport in violation of the restrictions contained herein or of the passport regulations is punishable by fine and/or imprisonment under 18 U.S.C. 1544. All statements and documents are subject to verification.

1001 may be construed as limiting the prohibition to “material” facts.

1542 has no such limitation, but prescribes punishment only for certain motives.

So, while the answer is “yes, you must,” others are correct in suggesting you might get away with it. But those who suggest you’re likely to regret doing so are also correct.

Lying on a passport application is a serious crime punishable by a fine of up to $250,000 or up to 10 years in prison. https://www.law.cornell.edu/uscode/text/18/1542

will there be any trouble?

No one knows. Maybe she gets away with it, maybe she gets caught. Even if it works today, she still can be found out later. Maybe in ten years when she renews. Maybe in 20 years Pakistan, India and the US have a shared marriage date base, who knows?

Given that it’s a serious crime with potential heavy penalty it is really NOT something she should do. Whatever the reason for hiding her status, she should consider alternatives in dealing with it. Talking to a lawyer might help: maybe her marriage can be annulled or can be considered "invalid" for US purposes.