Who is the copyright holder of a PhD thesis published in ProQuest?
The original author is probably the copyright holder.

Two other options: it is possible that it is the university/research institute (see, eg, this MIT policy for an example) but this is relatively unusual; alternatively, if it was done with industrial sponsorship, the company might own copyright. If either were the case, though, I'd expect to see some kind of indication on the thesis itself. Absent a clear hint (eg an existing university policy, a commercial-looking funder) that someone else would hold copyright, I'd work on the assumption it's the author.

You can try and trace them - this is not quite as impossible as it sounds, as the university alumni office may well have contact with them and be able to pass on a letter. If you have tried and failed, you might still have the option of using them without permission and an apologetic footnote...