There's no way for us to tell without more information. Some tachycardia is uncomfortable but not life threatening. Some tach can be deadly. Has the young man worn a heart monitor to record the arrhythmia? If so, then you and he can ask the doctor what this will mean for his future.
As i mentioned a friend has a son 16 years old suffering from tachycardia and chest pain after exercise, problem becoming worse. is tere a danger of sudden death?