Duchess in Diamonds by Jennifer Ashley

Eamon Stone, Hayden McCormick, and Dominic Wolfe have survived everything together—from a brutal schoolboy existence to deadly missions against Napoleon. Stranded behind enemy lines at Waterloo, they make a pact: if they live through the day, each will try to settle down and marry, leaving the last bachelor standing the loser of their wager.
No one expects Eamon Stone, son of a disgraced swindler, to fall in love first, least of all Eamon himself.

London, 1816
When Eamon calls on the widowed Duchess of Aylesmore to assess her late husband’s art collection, he hardly imagines she’ll be young, lovely, and in desperate straits. Caro Aylesmore, raising her nine-year-old son alone and drowning in her husband’s debts, hopes the paintings will save them.
Dragged across Europe by his trickster father, Eamon learned all about art, including frauds and forgeries. He hates disappointing the late duke’s beautiful second wife—so much so that he impulsively offers to search the entire house for anything of value.

He’s not certain why he’s being so gallant. But Caro’s quiet resilience reaches him in ways he can’t explain, and young Leo, the new duke, is already finding a place in Eamon’s heart.