It sounds reasonable, but I need more info:
What chemical will he be applying?
How will he be applying it (method)? Basal injections, soil drench, etc.
How many treatments, over what period of time?
In EAB treatment cost analysis, one thing we know for sure is that treating the tree is cheaper in the short term than removal and replacement. And in the long term, treatment is the best choice for preserving the environment benefits provided by such a massive tree. Plus, a tree that large could appraise at many thousands of $. If it were mine I'd have it appraised and insured.