Is it dangerous to have a tooth drilled if you have arrythmia?

Not reallly, No. Arrythmias or abnormal heart sounds or beats aren't normally dangerous. They can contribute to significant issues later in life depending on the arrhythmia. The thing is that some people over stress when the tooth is being drilled, which can cause an overwhelming amount of stress, which can affect the heart and potentially cause an arrhythmia to get so bad it turns into a fatal dysrythmia like Ventricular Fibrillation or Pulseless Ventricular Tachycardia (this ibex would be quite plausible), which cause cardiac arrest.