It has to be "with whom". "whom" is a formal word, and conversationally people mostly use "who" instead. However, the case "preposition + who" sounds wrong, and the conversational form requires a paraphrase.
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It has to be "with whom". "whom" is a formal word, and conversationally people mostly use "who" instead. However, the case "preposition + who" sounds wrong, and the conversational form requires a paraphrase. For example, "The woman with whom I lived" (formal) -> "The woman (who/that) I lived with" (everyday English).