Is the site irrigated?
The bush appears to be almost 18 inches from the grass. Using a Chemlawn gun you would have to be extremely clumsy to damage a bush so far from the grass. What kind of equipment were you using? Skid sprayer at 2 gal per K? Backpack? Ride-on? What pressure? What nozzle?
The most heavily affected part of the shrub is high on the sunniest part of the shrub. Why?
Did she spray the shrub for insects? Miracle grow? Jerry Bakers' formula? Shovel ice and salt off the porch?
Perhaps you should take a soil sample And try to grow grass in it in a flower pot (or coffee cup)--you should have an answer in 7 days. Plant a cup with clean soil for comparison. Save an extra sample for later chemical analysis, if needed.
Burning bush suffers from scale insects in a big way. And it does not tolerate poor drainage.