I am absolutely willing to post a negative review whether I know the person or not, if it is deserved. Not only does the business owner need to know what was wrong, but I have more than once been the unfortunate victim of going somewhere with only rave reviews and being horribly disappointed. Had there been negative reviews, I would have been more well-informed and possibly chosen elsewhere for dinner.
In my opinion, it is absolutely hypocritical to not post a negative review when one is deserved purely because you know the owner of a business. Unless you say something to them personally, and they remedy what caused it. But then the same holds true of a business where you know no one…if they fix it, then good for them. I’ll probably even mention that there was a problem and they fixed it in my review, because that shows excellent customer service. But to not say something is to do no one any favours, including your friend who runs the business. If they don’t know there is a problem, they can’t fix it. And you can’t count on anyone else to say something, because it’s a very real possibility that everyone will assume someone did with the end result being no one does.
That said, there is a huge difference between writing a negative review and being a snarky jerk. Negative reviews are helpful. Nasty, rude comments in the guise of negative reviews only show that you are the one who needs to make some changes, even when your point, if phrased differently, was a valid one.