John Gill Commentary 2 Samuel 13:3

"But Amnon had a friend, whose name was Jonadab, the son of Shimeah, David`s brother; and Jonadab was a very subtle man." — 2 Samuel 13:3 (ASV)

But Amnon had a friend
Though in the issue his friendship proved his ruin:

whose name [was] Jonadab, the son of Shimeah, David's brother ;
so that they were own cousins, the same with Shammah, (1 Samuel 16:9) ;

and Jonadab [was] a very subtle man ;
a very penetrating man into the dispositions of men, and could judge by their countenances what they were, and the affections of their minds, and had a talent in forming schemes; he was wise to do evil and was wickedly cunning.