My mom has a tooth infection with numbness on her face dentist said its normal. What should I do now?

Not normal. Get a second opinion from another dentist or see an ent. Numbness during an infection is not normal. I'd be very concerned about extension of the infection or malignancy.
Not always. Numbness over the face is not always normal. It could represent a tumor also. Have her seek a second opinion from another dentist, and insist on imaging studies. If not have her see an head and neck surgeon.
Dental Infection. Assuming you are a care giver you need to get the infection taken care of and dental treatment for her. At the start an antibiotic is most likely indicated.