I don’t know if you’re familiar with an online high end audio magazine called Mono and Stereo, but after reading quite a few of their articles, several years ago I emailed the editor and suggested that the publication would appear much more professional and polished if the English wording and grammar were corrected.
The owner, who also writes some of the reviews, is eastern European after all, and English isn’t his first language. He subsequently invited me to edit a couple of reviews, which actually ended up being nearly full rewrites. He was satisfied, and actually replaced his originals online with my rewrites, but when I asked him about the possibility of being paid for other editing jobs in the future, he demurred.
I really love editing and copywriting, which I studied for a while. Errors always jump out at me. Of course, I make my own like everyone else.