Jamieson, Fausset and Brown on Psalms 41:9

9. mine ... friend--literally, "the man of my peace."

eat ... bread--who depended on me or was well treated by me.

hath lifted up heel--in scornful violence. As David and his fortunes typified Christ and His (compare Introduction), so these words expressed the treatment he received, and also that of his Son and Lord; hence, though not distinctly prophetical, our Saviour (Joh 13:18) applies them to Judas, "that the Scripture may be fulfilled." This last phrase has a wide use in the New Testament, and is not restricted to denote special prophecies.