My mom 57 years old, has experienced burn sensation but only in her lips. Feels like eating something spicy she said. What is the diagnosa?

Reflux. It is possible that is she is experiencing reflux symptoms. See doctor for evaluation. This can be accompanied by laryngitis. A upper GI exam may be necessary to confirm the diagnosis.
Possibly BMS. Although reflux is certainly possible there is an entity called the burning mouth syndrome that is also possible. Burning mouth syndrome (bms) is a painful, frustrating condition often described as a scalding sensation in the tongue, lips, palate, or throughout the mouth. Although bms can affect anyone, it occurs most commonly in middle-aged or older women. Treatment includes drugs and local care.
Burning mouth. If she doesn't have a rash, it could be burning mouth syndrome. Many different causes with different treatments [dry mouth, nutrition problems like low vitamin b, mucosa/moist tissue problems, yeast, menopause, dry mouth, medications like ace inhibitors]. Talk with a doctor to sort out which cause and which medication might help her.