。
She wants someone to talk things over with, you know.
Literal
She [topic-は] consultation-partner [subject-が] is-wanted [explanatory-んだ][you-know-よ].
相談相手 ('a person to consult, advisor, sounding board') uses the productive noun 相手 ('partner, counterpart'), the same building block behind 話し相手 ('conversation partner'), 結婚相手 ('marriage partner'), and 遊び相手 ('playmate'). ~が欲しい ('want X') uses が as the marker for 欲しい's target — like 好き and 怖い, 欲しい takes its target with が rather than を. ~んだよ ends the sentence with explanatory ~んだ + emphatic よ — sets up the statement as informing the listener of a relevant fact ('see, the thing is...').