Is the use of the substance "Naffah," which is placed in the mouth and has an effect similar to smoke, considered unlawful (haram)?

If the substance in question functions as smoke, it is forbidden due to the corrupting effects and dangers it entails, which lead to perdition. A person must protect themselves and stay away from whatever harms them. As for your mother's persistence in using it, that is from the wiles of Satan. Therefore, she must seek Allah's help and make sincere repentance by immediately ceasing this disobedience, regretting it, and resolving not to return to it, especially since she has reached the age of seventy. She should consider the approaching end and departure, and beware of death surprising her while she persists in disobedience, lest she regret it.