@umbra21 - I think most people who put henna on their eyebrows will probably be doing it after using it on their hair. It sounds like your mother probably used extremely strong henna and maybe a kind that had another ingredient to give it a strong coloring.
Most commercially bought henna in the United States is going to have different levels of purity, depending on how it is sold. If you are following instructions from online, you are best off getting pure henna so it works properly in a given time frame.
But it's usually not going to cause an allergic reaction or anything, so, as long as someone is cautious, they probably don't need to do a patch test every time the way you should for normal hair dye.