The diagnosis of TB is suspected from a combination of context, symptoms, clinical signs and investigations. Mantoux  test is not confirmatory test for TB. Also sputum AFB negative sputum doesn't rule out TB.

Hi. Mantoux test is a screening test for Tb. While Sputum AFB test is confirmatory. Mantoux test comes false positive in many cases (most common reason being BCG vaccination) and has to be followed by a Sputum test for confirmation of diagnosis of Tb. Her sputum AFB testing is negative. So, she is not having Tb. If you have any doubts about reliability of the lab, you can repeat the Sputum AFB test at any government hospital or NABL certified lab.

For confirmation need to do a CB-NAAT