A person who is in a state of relapse can be very difficult to deal with. Addiction is the result of pain--either physical or emotional. The best and perhaps the only way to help that person would be to convince that person to seek the help of a professional. Meanwhile, you might want to be supportive by not being a lecturer as addicts hate being lectured to.
Addicts can be pretty cunning. They will use anything to their advantage if they so much as think it will enable them to continue in their addictive state of being. We all have failed at something at one time or another in our lives, so personally, I think the part about being a failure is just another excuse to continue using and abusing the substance of addiction.
Be persuasive… use whatever means necessary to get that person to seek help. If you love that person you must convince them to seek help, there is no other way to go about it.