After two years in the White House, one former senior official said, “she thought she was a senior adviser” — one who tried recently to weigh in on drafting Mr. Trump’s tweets — rather than an aide in a secretarial role. In recent months, Ms. Westerhout had become more interested in traveling with the president, and in Bedminster, it was noticed at a campaign briefing that she was seated closer to Mr. Trump than was his chief of staff…

Inside the faction-split White House, Trump loyalists cheered Ms. Westerhout’s departure as a move that was long overdue, and said they hoped it served as something of a wake-up call for Mr. Trump to bring in more loyalists into the West Wing. But current and former officials also expressed alarm about what information Ms. Westerhout could share down the road, not just about the president, but about her colleagues.

Adding to the concern was the fact that, unlike most other officials, Ms. Westerhout was not thought to have signed a nondisclosure agreement, a document that Mr. Trump has frequently used in effort to tamp down on leaks.

At least one publishing house on Friday had discussions about trying to approach Ms. Westerhout for a book, according to one person familiar with the discussions.