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.

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*_{I have a good relationship with professor but we never had any projects or publications together.}