No. The helicopter is not airworthy with any dents in a monocoque tailboom.
Most training helicopters, such as the Robinson 22/44, and Sikorsky S300, use a monocoque designed tailboom. With a monocoque design, there is no internal structure for strength. The strength is derived from the external structure or skin. Any dents in this structure would significantly weaken its overall strength. A common example is an aluminum soda can. The can is difficult to crush when there are no dents. You add a small dent in the can, and it can be collapsed with much less pressure.