What should a moderator do if a user posts content on a site that is well-meaning, but unfriendly in content?

A site should have clear policies so everyone knows what is appropriate and what isn't. In any case, you must follow your own policies. And it is helpful to continually consider and revise those policies when you notice they are not supporting the goals of your site.

With that said, the appropriate response (or policy revision) would depend on what you mean by "negativity". If you mean the user is giving well intentioned advice in a mean-spirited (or racist or sexist, etc) way, I think you discourage the mean spiritedness with warnings and/or bans if necessary.

On the other hand, if by "negativity" you mean that the user is stating facts and opinions without sugar coating, I think you should be pleased that your site is generating that kind of honest discussion. You can't please all the people all the time, so you are going to have to choose between pushing away those giving blunt advice or pushing away those that don't like hearing it.

You may also have to decide whether those flagging the post are potentially traumatized and needing protection from a well-meaning post that is actually harmful. Or if they are simply sheltered people, unable to handle legitimate facts and opinions that contradict their beliefs.

In any case, your choices need to depend on your goals... If your site generates revenue from people who prefer the sugar-coating, you might need to discourage honest and open commentary. If it is about advancing public knowledge, you are going to have to accept and encourage discussion that some will find offensive.

And if your main goal is increased user count, then regardless of the above you will want to avoid discouraging the person generating a lot of upvotes along with a few flags... That at least, is a simple numerical calculation.

Good luck.

As a past moderator on a forum full of debates, I had faced this case a lot of time.

As a moderator, the good reaction is, according to me:

- If the post is a question:

Edit the question with more "gap fillers" and all the needed friendly text. Put it clear that it had been added to the bulk of the question, but that the bulk is not changed, unless words in this bulk are clearly rude. It is important to edit and add those polite gap-fillers since otherwise, there won't be answers to this question.

- If the post is an answer:

For example if the user posted: "Your question is stupid, don't you understad/know X or Y"? Then you should post an answer close to this answer, which formulate differently what the user said in his answer. And have a direct message to the user saying: "Your answer is well meaning but you are to rude/unfriendly, please consider new formulations etc..." If there are truly rude or too familiar words in the answer, then edit the answer to delete those words. Otherwise don't, it's up to the user to change his answer.