The trouble with depression and anxiety is that they can appear to be very similar, unless you know what to look for. Even medications such as SSRI's will treat symptoms of either one, but they don't get to the root of the problem. While SSRI's manage the expression of symptoms, therapy can help to treat and uncover the cause of the issue. It helps to understand what the triggers are to these feelings and which behaviors actually feed into the feeling versus help you to cope.
This is a sensitive issue for the entire family, not just the person using. So many different dynamics come into play and the reality is that people suffering from addiction show much better progress when it gets seen as a family issue instead of someone struggling all alone. I have worked 7 years in substance abuse (Fresno, CA, Las Vegas, NV and in Wichita Falls, TX) and I have learned a lot about people's experiences, motivations, and struggles to stay clean.