Regardless of what insect is eating on this tree or what disease is harming it, environmental issues is usually a problem. Urban trees are subject to too much difficulty to stay healthy. They must compete with grasses and compaction to survive for many years.
The most common insect of any elm is the bark beetle....causing the ill fated DED disease, and the leaf beetle.
You can peer under the bark of a stem or small limb but peeling off the outer bark and looking for black or brown streaks in the inner wood.
I couldn't see a close up picture of the trunk as this is where to start as well as the surrounding drip line area. I often find where grade changes are made to the trees CRZ area. If you do not see any rootflares around the base of the tree, then the grade is wrong. Normally, when a tree, shrub,etc. starts dying from the top down, it is root related to the sap stream. Something internally is limiting the natural sap flow. Often, compact soils will cause toxic soil around the root zone or CRZ. IF the soil cannot breath to allow for natural gas exchanges, the root will die a slow death.
To rule out a perfect cause, take samples of leaves, twigs, roots to your extension service and have them tested if the customer is really curious. Otherwise, I see no need to render this tree to further distress, just have it removed and the stump cut out or ground. If 50% of the crown and surrounding leaves are gone, then take it down.