@bythewell - That's a shame for your mother. She should have gone to a hair dresser to fix her hair. They could have stripped back the henna and re-colored it something else.
Personally, I think people should stop coloring gray hair altogether. It's just a natural part of growing older.
I mean sure, if you like having your hair a particular color, go for it, but I think people get too anxious about what others will think of them if they show any signs of age.
Age should be an indicator of wisdom, or at least life experience. Not seen as some kind of weakness.