Professor reaction to sci-hub articles
I cannot tell you what authors would care in general, but I can tell you that most authors I know would not care at all. No one gets a dime from a paper anyway. 

I advise you however to not mention such a detail: it's actually irrelevant in that context, because you would be writing to discuss the content of the paper, not the ways to get access. When you write to someone, especially someone you don't know, avoid irrelevant details: people are busy.