Some causes of discolouration of the gums may be:

The mouth may have dark blue or blackish areas due to silver amalgam from a dental filling, graphite from falling with a pencil in the mouth, or a mole. Heavy cigarette smoking can lead to dark brown or black discoloration called smoker's melanosis. Ingesting lead or drugs that contain silver can lead to a gray discoloration of the gums. Minocycline, an antibiotic, discolour bone, which may show through near the teeth as gray or brown.
Addison's disease and cancer (such as malignant melanoma) can cause colour changes as well.

When in doubt...check it out!! Get her to see a dentist!

And definitley encourage her to quit smoking!! Every day she goes smoke free, give her kisses...she smokes and she goes without!