By what laws can the DoJ be the plaintiff for breaking confidentiality of the First Lady?

Question

Quoting Techdirt:

On Tuesday, the Justice Department sued Stephanie Winston Wolkoff, a former close friend of the First Lady, Melania Trump, who recently published a book about her relationship with Melania, including revealing phone conversations that Wolkoff had recorded.

The complaint says:

53. Under the common law, and in equity, Ms. Wolkoff had a fiduciary relationship with the United States of America based on her position of trust and confidence. Defendant served as an advisor on many aspects of the First Lady’s work for more than a year. She participated in confidential deliberations on sensitive topics, including on issues as varied as personnel decisions concerning the First Lady’s staff, the First Lady’s official initiatives, and communications with the media.

54. Ms. Wolkoff owes to the United States a fiduciary duty to protect confidential information in her possession that she obtained through her service as an advisor to the First Lady.

What are the specifics of common-law and equity that allow the DoJ to be the plaintiff?

Answer 1

The DoJ is the USA

Or, more precisely, the attorney general (the head of the DoJ) is the member of the cabinet that represents the United States Government in legal matters.

The US government is a legal person and can sue and be sued (subject to sovereign immunity), just like any other person. When they do, the attorney general through the department of justice is the natural person who does it.

Now, if you are asking what the particular merits of this particular case are: that’s a different question.