。
After reading the letter, she ended up tearing it up.
Literal
She [topic-は] that letter [object-を] read [after-あと] tore-up-regrettably.
~あと ('after [verb]') marks sequential timing: 'after reading, [next thing happened].' 破く ('to tear, rip') describes the destructive action; ~てしまう adds either a sense of completion or, more often, a flavor of regret or inevitability — the action happened in a notable or possibly regretted way. The combination here suggests an emotionally charged moment: she read it, and then the contents drove her to tear the paper. The whole is the kind of sentence a narrator uses to mark a turning point.