After driving a few minutes the lights come on and the car starts stalling, HEV warning and brake warning lights

After driving a few minutes the lights come on and the car starts stalling. My mechanic says it is the lithium-ion battery that needs to be replaced. I live on a Caribbean island and there is not Infiniti dealer located here. Just looking for advice about what I should ask the mechanic before spending $5,000.00 to replace the battery. The car has been maintained and is perfect condition with very low mileage.