It's rude if it's done in public. But please, if a poster's feelings are hurt by being corrected in an anonymous forum ... well, then he or she needs to grow a thicker skin.
I don't correct anyone's grammar. But some grammatical errors are just too dumb to tolerate. The apostrophe abuse committed by writing the word "Kennedy's," for example, where a simple plural was indicated, is one such error.
The FReeper who posted the correction was performing a service. And the poster who was corrected accepted the correction gracefully. All is well.
Yoor nerodic. get halp befor yu dryv insayn everwon around yu, and there frends to.