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

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.