It is difficult to find the cause of rapid pulse or
tachycardia on net.
Tachycardia can be due to low BP, anemia, hyperthyroidism, fever, low blood sugar, anxiety and panic disorder, heart causes like sinus tachycardia and palpitations, alcohol, coffee, effect of certain drugs etc. Hence, if she takes too much caffeine or alcohol, then this needs to be cut down. Also you need to consult a doctor to run tests to rule out other causes. Take care!