UPDATE 2: More information from The Washington Post:
“Roger’s one of my best friends,” Davis said in a telephone interview Friday. “I have nothing bad to say about him.” She called the Mueller probe a “witch hunt,” and said “in terms of Russian collusion I know nothing.”
(...)
Davis is the latest of multiple current and former Stone associates to be contacted by Mueller’s team. His longtime associates Sam Nunberg and Michael Caputo — both of whom held posts in Trump’s 2016 campaign — have said they were questioned about Stone. Jason Sullivan, who briefly worked as Stone’s social media coordinator, appeared at D.C. federal court after receiving a subpoena. And Stone’s driver, John Kakanis, has also been subpoenaed.
(...)
Another former Stone aide, Andrew Miller, is seeking to quash a subpoena he received from Mueller’s team. Davis said she was unsure whether she would attempt to do the same.
Davis, a former hedge fund executive, was known for running a high-end prostitution ring in New York City. She has served time in prison on prostitution charges and a drug charge.
In 2010, Stone was a strategist for Davis’s campaign for New York governor, a race in which she garnered more than 20,000 votes — less than 1 percent of the total ballots cast. In the years since she has become one of Stone’s closest confidants, tending to his travel and speaking schedule.
Mueller seeking to question ‘Manhattan madam’ in Russia probe, she says (WaPo)
UPDATE: NBC News confirms:
According to TMZ, Special Counsel Robert Mueller has issued a subpoena to Kristin Davis, the "Manhattan Madam," because of her connection to Roger Stone.
Davis worked for former Trump aide Roger Stone for a decade, and had numerous interactions with Stone and Andrew Miller -- who ran Davis' campaign for Governor and who was subpoenaed by Mueller a month ago.
Sources familiar with the matter tell us, Davis' lawyer is negotiating the scope of the subpoena with Mueller's team.
... Stone was referenced in the indictments last week of 12 Russians who allegedly meddled in the 2016 election.
MANHATTAN MADAM KRISTIN DAVISSUBPOENAED BY ROBERT MUELLER (TMZ)