Insān al-ʿayn can denote the pupil of the eye. The term “drowning” treats heavy tears as water in which that “person” could be submerged. The poem creates not only exaggeration but an apt connection between attributed cause and quality.
Three elements matter: the informer’s offense is normally harmful; weeping belongs to the emotional background; and “saving” the eye’s pupil is the poet’s invented explanation. Reducing the poem to “the informer is bad” misses the figure entirely.