The answer to What do you do about a single offensive sentence? as evidenced by the answers and comments is that are multiple options. None of the options given are "wrong" and none are better then the others. Some people prefer different approaches. Choosing any of the choices presented here, is fine (even the ones that I personally would not choose)
We are a community and no one person should feel they are responsible to make the "right" choice on behalf of the community. If your not a mod, the available options are all easily reversible. Even if you are a mod, most things you do can be easily reversed by the community.
- Flag it for a moderator
- Leave a polite comment
- Edit the Post
All of these are valid options, if part of a post makes you think something needs to be done, do it. If others don't agree they have other options.
There are two things you should not do
- Do not: Have an edit war, if someone other than the OP edited the post in good faith, and you are not the OP, don't revert without a discussion in meta.
- Do not: Have a prolonged discussion in the comments, when you start getting those popups take it to chat or bring it to meta.
Act in good faith, don't respond in anger, and do the best you can.