Can anyone offer any arguments for or against the mandatory wearing of safety helmets?
This seems like a good case of the individual in question being the only one in danger, so why go beyond informing them of the risks? We could say that allowing idiocy might influence others to emulate it, or that the cost and upset of a death or injury affects many others, but such an approach would effectively mean the complete repudiation of the harm principle.
And, similarly, what of attempted suicide? Is that different because it may be (is?) the result of mental ill-health rather than some hypothetical free-will?