“a friend whose name was Jonadab.” Although Jonadab was Amnon’s cousin, in this context it was their friendship that was important. In fact, often in royal families, the cousins were rivals and more enemies than friends. This record emphasizes how important it is to choose friends wisely. This “friend” ultimately cost Amnon his life.
“Jonadab, the son of Shimeah, David’s brother.” In 1 Samuel 16:9, Shimeah is called Shammah, and was David’s brother, the third son of Jesse. So Shimeah is Amnon’s uncle and Jonadab is Amnon’s cousin.
“shrewd.” The Hebrew word is chakam (#02450 חָכָם), and it can mean “wise, experienced, shrewd, cunning, crafty, etc., depending on the context. Jonadab was all those things, but in this context, shrewd, cunning, or crafty would fit well. It is not godly or wise to give someone bad advice. Perhaps Jonadab thought if he could help Amnon get what he wanted then Amnon, who was David’s first son and thus was in line to be king when David died, would someday give him power and authority in the kingdom.