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She's the kind of person who'll generously give anyone whatever she has.
Literal
She [topic-は] anything generously people [to-に] give person is.
気前よく is the adverbial form of 気前がいい ('generous,' literally 'spirit-front is good') — describing someone who spends or gives freely without reluctance. The whole modifying clause なんでも気前よく人にやる describes 人 ('person'), with 彼女 then identified as that kind of person via the [N]+は+[modified N]+だ structure. やる here is the rough/casual equivalent of あげる ('give'), used especially for giving 'down' to peers, animals, or in characterising contexts; it carries no rudeness in this descriptive use.