Is it unethical to send unofficial feedback to authors of a paper which I declined to review due to conflict of interest? I was asked to review a paper which was submitted to a conference. The author of the paper was one of my former professors* and of course I declined to review the submission due to conflict interest.

I was wondering however, would it be unethical to write an email to my professor give him a feedback on the paper and perhaps, provide him some kind of unofficial review? 

Of course it would not change the fate of the paper but if the paper gets accepted, they can use my pseudo-review when doing their final editing. I was not sure if this is ethical and/or according to the etiquette of academia.

Edit: Perhaps I should clarify what do I mean by feedback. What I had in my mind wasn't a complete review but rather some suggestions. Something in the lines of:


  Very interesting stuff bla bla... however, I think it would be better
  if you had written sec. IV in this fashion and you add data-field foo
  to figure bar.




*I have a good relationship with professor but we never had any projects or publications together.
There might be an ethical problem with telling the professor that you were asked to review his paper, but I think it's a minor issue: the anonymity of the reviewers is mostly there to allow them to give their opinion freely. If you are eventually not reviewing the paper then I don't think you are required to preserve that anonymity (even if you do review the paper, it's not totally clear that you must stay quiet about it). 

And if the paper is available online on some archive or on the authors' web pages, then it is definitely ok to give the authors your opinion based on the online version if there is a plausible way in which you could know about the paper had you not been asked about it.

I feel that you main concern is that the fact that the paper was sent to you in error would give the authors an unfair advantage by giving them the opportunity to hear your opinion about it. Well, it is unfair, as are many things in life, but not un-ethical. Ethics is there to keep science working well and making progress, and by letting the authors know your opinion you are doing exactly that. Fairness is only relevant when there is competition for competition's sake, and science should not be that.