What can i do if my sisters stressing about a flouride stain on her teeth, will it go?

Things can be done: Fluoride stain (fluorosis) does no go away. It makes the teeth look grey and like granite. Sometimes a nice improvement can be made with whitening trays over a long period of time. The quick fix is veneers.

Enamel Stain: Can also be caused by metabolic disorder, trauma, etc...In case of mottled enamel and pitting, a resin restoration that matches the teeth can replace the weak or damaged part of the tooth.

Fluoride stain?: Does this stain effect only one tooth or multiple teeth? Did the teeth come in this way or did it happen later? Damage from excessive Fluoride would have to be present from the time of eruption and effect multiple teeth. If not then this stain is not the result of Fluoride but from something else. In any case a dentist can offer her options for restoration.