A) Ignore it and simply help out the original poster, thus providing a good example for other forumers.
B) Consider that they possibly may be quite ill and acting out of character and proceed to option A.
C) Draw attention to the mistake by making a post dripping in sarcasm and generally act like a jerk.
D) Simply help out the original poster, and briefly mention to the offender what could be done next time.
You had pretty good odds there, only one option was wrong.