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.

Reference(s):

Aircraft POH

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