There are various causes of tachycardia. These include heart disease, high blood pressure, thyroid disease, electrolyte imbalance, medications, caffeine, alcohol, smoking, or other recreational drug abuse, and psychological stress or anxiety. It is important to consider and treat the cause of the tachycardia, rather than just treating the condition. Simply slowing the heart rate could cause more harm if the rapid heartbeat is a symptom of a more serious or long-term problem. Encouraging her to quit/taper smoking and eat healthy are good ideas. Bananas can help replenish potassium if hypokalemic, but make sure she is not hyperkalemic because electrolyte imbalance in either direction can cause arrhythmias.