I hope that I'm getting on the same page as you. I'm really trying.
If this particular person contacted me privately to let me know they've read the story and found X problems, instead of the negative review. I would look at the parts in question, see if there actually was a problem, address it if needed, and then reply back with a customer service style response of "thanks for letting me know" etc. Authors have done that to me before and I always look into it.
The core of my issues with the review and random comments about grammar/punctuation is that they're uncalled for. If there was something terribly wrong with it, privately message me. I would happily take a DM or a mean review. A typo or a misplaced comma doesn't make the story incomprehensible. You wouldn't review a traditionally published book lower because a typo slipped passed the editors or the writer had a different style of English. But a lot of readers seem to think with the rise of indie published work that they have to pick everything apart.
I would rather hear people liked a particular story arc or that they related to the characters over comments about typos. I know that those kinds of reviews "should" come with time but I'm impatient. haha.