A friend was just diagnosed with Dementia. She made a hurtful statement to a mutual friend. It only happened once about 7 weeks ago.

Probably. Social filters are controlled by a specific section of the brain, and when they are affected by dementia they can stop working so well. If a normally kind and tactful person suddenly blurts out rude, hurtful, personal comments in public you would certainly be suspicious - especially if they don't seem even to be aware that there's a problem.

You may find this happens more as time goes on.