。
She ended up losing to him in tennis.
Literal
She [topic-は] he [to-に] tennis [in-で] ended-up-losing.
負ける ('to lose') takes に for the opponent — losing is conceptualized as an action directed 'to' the winner. The auxiliary ~てしまう adds a tone of regret or unintended completion: she didn't mean to lose, but she did. The で here marks the activity ('tennis') within which the outcome occurred.