We can not give you a definitive answer here, it totally depends on her situation and the best person who can give such an advice is her doctor.
If she is a person who has a history of a sudden health problems related to hypertension or any other sickness, it is best to pay a visit to a doctor to assess her overall health situation before flying.
Anyway, let's assume that she is in a stable situation and she has her meds with her, then there is nothing to worry about. 2.5 hrs is a short flying time and I am not aware of any negative sides of flying on people with hypertension. I see them flying all the time and I know that because of the number of low sodium meals on each flight (cabin crew member here).
One more thing, do not forget to make a special order of a low sodium meal in case the flight has some sort of a meal service.
Finally, each flight has a Doctor Kit, which can be opened by any doctor onboard in case of health emergencies. The kit has various medicines for common health problems that require immediate intervention, including allergies, diabetes, heart attacks and hypertension. Hopefully no one needs that.