The answer may be given from several angles:
First, they must establish the authenticity of the report. Merely attributing it to a narration by Abū Nu‘aym does not(1), by consensus, constitute proof. Abū Nu‘aym has a well-known book on “The Virtues of the Companions” (2), and he also mentioned a selection of their virtues at the beginning of al-Hilyah. If they invoke his narrations as proof, he narrated concerning the virtues of Abū Bakr, ‘Umar, and ‘Uthmān that which demolishes their edifice and tears down its pillars. But if they [do not](3) invoke his narrations as proof, they cannot rely upon his transmission.
As for what he and others narrated, we refer it to the experts in this discipline and to the methods by which the truth or falsity of a hadith is determined. This entails examining its chain of transmission and its narrators: whether they are reliable and whether they heard from one another. We also examine the hadith’s corroborating reports and whatever indicates, [regarding it](4), either of the two conclusions. We make no distinction between reports concerning the virtues of Alī and those concerning the virtues of anyone else. Whatever is established as true, we affirm as true; and whatever is false, we reject as false.
Thus, we bring forward the truth and affirm it: we neither lie nor declare a truthful person a liar. This is well known among the leading scholars of the Sunnah. As for anyone who fabricates a lie against Allah or denies(5) the truth, we must declare him false in both his lying and his denial of the truth, as with the followers of Musaylimah