פּירוש אויף קהלת 4:10

For if they fall. [To be understood] according to its apparent meaning. And regarding Torah studies, if his studies are too difficult for him, his companion will review them with him, or if he errs and is not exact in what he heard from his teacher, his companion will set him on the truth[ful course].¹⁴The Gemara in Maseches Berachos 63b states that Torah scholars who sit alone and engage in Torah study become foolish and will eventually sin [by erring].

But woe to him. But woe to him וְאוֹי לוֹ=וְאִי לוֹ.¹⁵Rashi renders this interpretation because ואילו is spelled ‘full’, i.e. with a yud. (Sifsei Chachomim)