the lawyer; the clientThis is not a fixed phrase. 弁護士は means “the lawyer” as the topic, while 依頼人 means “client” and is followed by another particle or phrase, such as 依頼人に (to the client) or 依頼人の (the client's).
JA: 弁護士は依頼人に詳しく説明した。
EN: The lawyer explained it in detail to the client.