The watchman, as his duty was, had sent immediate notice to the king that he saw a crowd approaching (see 2 Kings 9:17-20). Jonadab, who was with the king, was prompt to give the explanation.
Matthew Henry
Concise Bible Commentary
Jonadab was as guilty of Ammon's death, as of his sin; such false friends do they prove, who counsel us to do wickedly. Instead of loathing Absalom as a murderer, David, after a time, longed to go forth to him. This was David's infirmity: God saw something in his heart that made a difference, else we should have thought that he, as much as Eli, honoured his sons more than God.
SDA Bible Commentary (limited)
Volume 1-7
35. As thy servant said. When Jonadab had previously told David that only Amnon was dead (v.32) he was evidently speaking, not from a knowledge of the facts, but from a shrewd surmise. Seeing the princes approaching, he knew that he was right, and did not hesitate mentioning this to David.