Receiving a reviewer comments of the wrong paper from editor
Don't overthink this. Write a concise and polite reply. Here is an example that is not perfect but should do it.


  Dear Professor ...,
  
  we recently received the reviews for our paper ...., which we
  submitted to the journal ...., of which you are a/the editor of.
  
  While we found the comments by reviewer 1 very helpful, we noticed
  that review number 2 was written for a different paper. We'd like to
  hereby inform you of this likely administrative error. It is our
  understanding that there is a second review for our paper, and hope
  that its content will be equally helpful to improve our paper.
  
  Thanks,  Author name


If there is a submission number for your paper, include it as well (perhaps also submission date).