DBP
Is your friend trying to get both apples and plums off the same tree? I see an obvious problem there.
The best thing you can do to protect any plant, organically speaking, is to simply to keep it healthy. A healthy tree will always do better at discouraging pests than a struggling one.
♦ Fruit trees love sun. If your Italian Prune is being shaded out, prune back the branches of other trees that are shading it, if you can. Prune back any older branches that are not producing fruit at all. (Those branches are not getting enough sun.)
♦ Dress the base of the tree with plenty of light mulch in the spring, and water it regularly throughout the growing season. Make sure there is plenty of light ground cover (grass or shallow-rooting flowers) outside of the drip ring, so the tree’s extended root network is in cool, moist soil.
♦ Look for and remove any other stressors that might be compromising the tree’s ability to protect itself.