John Gill Commentary 2 Samuel 15:31

"And one told David, saying, Ahithophel is among the conspirators with Absalom. And David said, O Jehovah, I pray thee, turn the counsel of Ahithophel into foolishness." — 2 Samuel 15:31 (ASV)

And [one] told David
That came either from Hebron or from Jerusalem:

Ahithophel [is] among the conspirators with Absalom ;
Absalom sent for him, and it seems he came to him, and continued with him, see (2 Samuel 15:12) ;

and David said, O Lord, I pray thee, turn the counsel of Ahithophel
into foolishness ;
either suffer him to give foolish counsel, or confound the schemes projected by him, and let them not be carried into execution; for God can, and sometimes does, disappoint crafty counsellors, that they cannot perform what they devise, but they are taken in their own craftiness, and their counsel is carried headlong, (Job 5:12Job 5:13) ; this prayer was answered, (2 Samuel 17:14 2 Samuel 17:23) .