Who said unto God, Depart from us (comp. Job 21:14). Eliphaz tries, though with no very great success, to turn Job's words against him. And, What can the Almighty do for them? i.e. and ask what the Almighty can do for them.
A change from the second to the third person, without any change of subject, is not unusual in Hebrew. The wicked renounce God, and bid him depart from them—conduct which they justify by asking what good he could do them if they acted otherwise.
The idea is the same as that of Job 21:15, though not expressed so pointedly. What Eliphaz thinks to gain by echoing Job's words is not very apparent.