We can never understand or judge the reasons for someone elses suffering, we can only try to look at what our own lessons are.
If you truly believe that everything happens for a reason, and I do, since I was a very young child, long story, then all there is to do is live your own life authentically, when you are in pain, look at it, pain is a sure sign that something is amiss, it is also the only way to enlightenment. We can only have an understanding of our own enlightenment, not someone elses. If they share it with us, then we are closer to understanding. We do not live spiritually in duality with someone else, we may learn and grow together, but our journeys are very singular.
Self Love is always the end goal. You have to trust that somewhere deep down inside, the suffering of your addict is the pathway to his enlightenment, and appreciate the fact that his suffering has helped you to become more in touch with yours.
My take anyway, maybe I haven't expressed it well, but there it is.