She was betrayed by her friend and is plotting revenge.

Literal

She [topic-は] friend [by-に] was-betrayed, someday revenge do [volitional-と] is-thinking.

裏切られ (passive of 裏切る, 'to betray') + いつか仕返ししてやろうと思っている ('is thinking of getting revenge someday'). ~てやろう uses the casual benefactive やる with volitional, adding a determined/vindictive tone.