Im not sure it's unfair given the poster said they bought it new! Their issue is the sellers apparent refusal to comply with their legal rights.
It shouldn't self destruct. Any mechanism should be protected by shear bolts to stop that happening, even our cheap topper has them.
If the seller refuses to refund the buyer could take them to small claims as not fit for purpose.
Now it might be that the buyer would have been better off with a flail mower if the ground is stony etc, but using a normal one shouldn't result in this damage on first use really.