How can I provide proof that my paper has been proofread by a native speaker?
This seems like an unreasonable request. Frankly, it does not matter if you paper has been proof-read by a native speaker or not. If, after revision, your English presentation is still bad, the editor won't accept it even if you include proof that it was read by a native speaker.

I would personally be prone to just ignore this request, make sure that the English language is as good as it can be, and resubmit the paper.