Found and corrected a mistake on someone's else paper -- praxis?
I have not been in such situation. However, I do not see what is the problem for you to write a paper complementing or correcting the previous paper? Isn't this how science works?

In my opinion, it is very toxic culture in academia to consider such thing as inappropriate. Those previous auathors are humans. Assuming good faith, that was what they knew and what was to the best of their knowledge, and to the best of the reviewers' knowledge, at the time of their publication. You got something, great, you have the right to get the credit for it. I feel it is unfair that you inform the authors offline. You should write it, and publish it too. 

Also, we should not forget that you might be wrong too! I do not mean to offend anyone. But if we remember that we are all humans and have limitations in our knowledge about exploring this world, we would take these issues in a more relaxed way.