[TalnSG]

It won't do any good to confront someone like that. They will blow it off and it will probably fuel more bad-mouthing. Aside from people with authority over you, any effort to even address it is usually wasted except if it alleviates yoru own frustration.

Yes, if its an employer, teacher or other person with actual (not just perceived) authority over you address it immediately, vigorously and in specifc detail.

Otherwise, consider that anyone who would believe the sort of person you are describing has as little to offer as she does when it comes to friendship or intelligence. If someone mentions it to you tell them to "consider the source" and make their own judgement. It is extremely difficult, but if you can ignore it you will be far better off. It's called rising above them,