Albert Barnes Commentary Job 5:16

"So the poor hath hope, And iniquity stoppeth her mouth." — Job 5:16 (ASV)

So the poor has hope – from God's intervention. They are not left in a sad and comfortless condition. They are permitted to regard God as their protector and friend, and to look forward to another and a better world. This sentiment aligns with all that is said elsewhere in the Scriptures: that the offers of mercy are specially made to the poor, and that they are especially the objects of divine compassion.

And iniquity stops her mouth – that is, the wicked are confounded when they see all their plans foiled, and find themselves entangled in the snares which they have laid for others. A similar sentiment occurs in Psalm 107:41-42:

Yet setteth he the poor on high from affliction,
And maketh him families like a flock.
The righteous shall see it and rejoice,
And all iniquity shall stop her mouth.

It should be remembered that Eliphaz states this as the result of his own observation, clearly demonstrating, in his view, that there is a superintending and overruling Providence.

A careful observation of the course of events would undoubtedly lead to the same conclusion, and this has been embodied in almost every language by some proverbial sentiment. We express it by saying, "honesty is the best policy" – a proverb undoubtedly founded in wisdom.

The sentiment is that if one wishes for long-term prosperity, one should pursue a straightforward and honest course; that cunning, intrigue, underhanded dealing, and mere manipulation will sooner or later defeat itself and recoil upon the one who uses it; and that, therefore, even if there were no higher motive than self-interest, one should be honest, frank, and open.

See this argument stated at greater length, and with great beauty, in Psalm 37.