。
Everyone I know speaks well of her.
Literal
I [subject-が] know people [topic-は] all her [of-の] matter [object-を] well say.
僕が知っている人 is a relative clause modifying 人 — '[the people who] I know.' Note が marking the internal subject of the relative clause. 良く言う literally 'say well [of]' is an idiom for 'to speak well of, to praise.' 彼女のこと marks 'about her' as the topic of speech.