A dr said my girlfriend has optic neuritis, and need immediate medical care, does she have MS for sure? Her aunt has ms.

Statistical risk. According to data from optic neuritis treatment trial, risk of clinically definite ms, if MRI lacks any lesions is 25% by 15 yrs, but, if only one white matter lesion, the risk escalates to 72%. Therefore, get followup MRI studies, and find neurologist who focuses in ms to work with. Please do not worry, newer meds are far more successful these days. Do supplement vitamin d-3.
See your eye doctor. Up to 50 % of ms patients have visual symptoms or optic neuritis as a presenting sign, however that does not mean your girlfriend has ms. There are many causes of optic neuritis, all of which should be evaluated and treated by your eye doctor or neuro-ophthalmologist. Due to her age and family history ms is high on the differential, she should have an MRI of the brain to further evaluate.