You can't force someone to get treatment, but you can make it clear to them that there are consequences for their behavior. Point out to them the negative ways their addiction is affecting their life, and let them know how it affects you. It might help to get other people involved as well, to help make your point.
Research has actually shown that addiction treatment doesn't have to be voluntary to be effective. For example, court-ordered addiction treatment works even if the person is only doing it because they have to. So you don't necessarily have to convince the person they have an addiction--even if you can just convince them to get treatment for your sake, it might help.
Click Here to search for a treatment center in your state that is right for you.