Another Woman's Husband / Gill Paul.

Two women, divided by time, bound by a secret...1911. Aged
just fifteen, Mary Kirk and Wallis Warfield meet at summer
camp. Their friendship will survive heartbreaks,
continents, and the demands of the English crown, until it
is shattered by one unforgivable betrayal...1997.
Kendall's romantic break in Paris with her fiance is
interrupted when the taxi in front crashes suddenly. The
news soon follows: Princess Diana is dead. Trying to
forget what she has witnessed, Kendall returns home, where
the discovery of a long-forgotten link to Wallis Simpson,
the Duchess of Windsor, will lead her to the truth of a
scandal which shook the world...Richly imagined and
beautifully written, ANOTHER WOMAN'S HUSBAND is a moving
novel about two women thrust into the spotlight, haunted
by love and loss.