What standards should I apply when evaluating a PhD thesis from a weaker university?
I suggest that you remember that you are evaluating the work, not its author, or the supervisor, or the university. Give it an honest evaluation, based only on what you see before you. If it doesn't measure up to your standards, then say so. Say why. Make suggestions if you have the time. But honesty is required.
One of the reasons, actually, for international reviewers is to give the home institution some assurance that they are producing students on a par with others.