Someone takes a cardiac treadmill stress test and experiences tachycardia which is then identified by an cardiologist as a benign tachycardia.problem?

No: Benign means "not harmful".

Difficult to say: Based on the information provided, it's difficult to answer your questions. As while a tachycardia may be benign - as in not life-threatening - it doesn't mean that it won't cause symptoms like palpitations or lightheadedness. It may even further evaluation and possibly even medications to control. If it is sinus tachycardia, which is expected to be found on a stress test, then I wouldn't worry