In a comment made on an answer, a user has asked for clarification on the provided answer. Someone has responded, but the advice they have given is incorrect in my eyes, and so I have provided what I beleive to be the correct answer to the users query about the answer. I can't downvote the incorrect answer, though, as it's a comment, and only on a minor point of clarification. Should I just leave it be, and hope that other users see a my response? I wouldn't classify it as "noise, offensive or spam" so I'm reluctant to flag it as such.
You've corrected the comment with yours.
The only other thing you could do would be be to address the commenter directly - using @username - to let them know. They'll get notification of the comment and it will be then up to them to edit (if it's within 5 minutes) or delete the comment.
If the comment is "dangerously" wrong then you should flag it and a moderator could delete it if they agree.