“Even my close friend.” This “close friend” is not identified in the Psalm. A number of commentators think that this refers to David’s close friend Ahithophel, but that cannot be substantiated. The fact is that being betrayed by a close friend is such a common occurrence that it is more of a universal truth than a specific event, even though David may have had a specific friend in mind. David was betrayed by a close friend, and Psalm 41:9 not only applied to David, but to the Greater David, the Lord Jesus Christ, who quoted it at the Last Supper (John 13:18), and in that context, it referred to Judas Iscariot.
“has lifted up his heel against me.” The phrase “lifted up his heel” is an idiom for turning against someone and betraying them. Jesus referred to this psalm at the Last Supper in reference to Judas Iscariot (John 13:18).