His Lady Deceived. The Deceived Series, #2

Lady Sarah Milton's spinsterhood is not because she hasn't received proposals of marriage. Her beauty and dowry have attracted many. It's just that the men who have proposed don't appeal to her. One man who does, the sport-mad Alfred Wickham, doesn't know she exists. Therefore, when his mother invites her family to spend Yuletide with her extended family at the home of the Duke and Duchess of Radcliff, Lady Sarah jumps at the chance. Alfred Wickham is not happy.
His matchmaking mother fancies an alliance between him and some well-bred young miss he's to meet at the Radcliffs'. The last thing Alfred wants is to tie himself down in matrimony--and, besides, he's never been attracted to well-bred ladies. He'll show them! He enlists his best friend, Lord Pottinger, to intervene. Potts is to inform the miss of Alfred's wicked ways: his gaming, his recklessness, and his (feigned) understanding with an actress.
Once Potts has succeeded in poisoning Lady Sarah against his friend, Alfred realizes he's made a most dreadful mistake .