My friend is 18 and her heart rate increases rapidly when she stands up what could be wrong?

Diet Active Symptoms: First, does she have symptoms when this happens - dizzy, palpitations, sweating? How is her diet? Dehydration and/or poor nutrition might be a cause. What happens when she exercises - fatigue, other symptoms or no problem? Two rare problems POTS and inappropriate sinus tachycardia are considerations. I would recommend seeing her doctor and review her complete story to determine next steps!