Just re-read it and you're right, Daniko; the interview doesn't identify him.
Another reference which I have a copy of, "P-38 Lightning Aces of the ETO/MTO", states that Graham was Heiden's flight leader on that mission. From page 74:
"The 77th FS ran into Bf 110s on 18 March and shot down three of them. These victories were not without cost, however, as P-38 ace Capt. Lindol Graham was killed in action. He and his wingman, Lt Art Heiden, were hard on the tail of an evading Zerstorer when the pilot put his twin-engined fighter down in the snow. Heiden then watched in disbelief as Graham made a pass at the fleeing German crew, misjudged his altitude and clipped the snow-covered ground, before turning over and crashing."
The same book also identifies the fighter which shot Morris down as an Me 410:
"The seven remaining victories scored on 7 July fell to the 20th FG, although they lost a single P-38. That latter happened to be none other than the ranking Lightning ace in northern Europe, Capt. Jim Morris, who was in the process of shooting down an Me 410 when the fuselage-mounted rear guns of the mortally damaged German aircraft hit his P-38 and forced the ace to bale out."