In that situation it is your place to intervene because the person has his or her willpower and logical mind hijacked by alcoholism. We are creatures of logic but also of passion. Sometime our body will do things that our mind don't want them to. In those situation, help is required. The person can't defend itself so it's your place to help them.
Sure but people still have bodily autonomy. Nobody has the right over anyone's body. You can tell them the risks, you can try to convince them to get rehab, but they ultimately have the right to do what they want with their body. Whether or not you disagree with someone's right is irrelevant, it's still one of their inalienable human rights.
It's really not that simple and you're confusing things. Sure, we have the right not to be molested or have treatment forced on us, but that's completely different from the idea of having the right to self-harm. There is a common fallacy, that we have a moral right to do whatever we wish with out bodies. We don't. This has nothing to do with anybody else having a right over anyone else or the law (we must distinguish between MORAL rights and legal rights, or I could just as easily say that legislation could change the right you believe you have). Even if you were the only person left on the planet, you would not have the moral right to self-harm. Why? Because we do not have the right to moral evil and self-harm is a moral evil.
Moreover, unless those engaged in genetic experimentation sterilize themselves, it also poses risks to population genetics.
I'm not actually confusing anything, and I don't see where your definition of "Moral evil" is coming from.I don't see why the right to self-harm is immoral. Tattoos and piercings are self-mutilation. I agree on the risk of population genetics and think that sterilization should be required before even attempting such a procedure, but that's because you're risking the well being of others, not self.