A person claims to have earned a PhD from Harvard University, but is offering no credible proof. How can I verify their claim?
Harvard has a page on their website dedicated to this exact question, and the answer turns out to be simply "contact the specific school and request verification of the degree."
To speak to the general case, just call the university registrar and ask how they go about verifying degrees. This is a very common scenario and they will almost certainly have a process in place for it.
Research universities tend to archive their PhD dissertations and list them in the online library catalogue. The institution named in the question does that.