You're absolutely right about legal liability. They have every right to sue. But as to the discussion of who's at fault in terms of preventing the accident, ye

You're absolutely right about legal liability. They have every right to sue. But as to the discussion of who's at fault in terms of preventing the accident, yes, it's the friend's sister's parents. The friend's sister was still too young (13) for that e-bike which the company rated for age 18 and up. I'm not speaking from the standpoint of how the law ought to work. I agree that companies ought to provide products fit for use and fit for purpose. I agree the parents should be able to sue if the product isn't. The helmet being defective is a much worse problem, if that's true. If we're talking about whose job it is to prevent children from getting involved in an accident of this sort in the first place, however; that's on parents.