Antidepressants and alcohol might not interact with each other, that in no way makes them all completely safe. Both have significant effects on your brain chemistry, its rather stupid to blindly think taking one drug to drastically change your brain chemistry in one direction then taking another to swing it the other way is a safe thing for your brain.... it's not.
The guy need's help, get him to talk about it when sober, tell him he has to stop drinking. Ask him to think about if his life is really improved when he drinks, can he remember anything or is he getting black out drunk. Is he just happier to be black out drunk than just being unhappy, thats a pretty big reason for a lot of depressed people to drink. Better to feel nothing, than depressed. however get him to see that he might feel better when drunk, but he's actively hurting his friends and his relationships with people. He might not feel so bad when drunk, but he's going to feel worse when sober till it gets to the point no one will talk to him when sober.
Hopefully he'll see sense and realise drinking doesn't help him and also doesn't give his brain a chance to "normalise" which is what anti-depressants try to do. Assuming he's not feeling any better on them, and has erratic sleeping and by the sounds of it no improvement on the pills he needs to speak to his doctor about it, maybe get a different pill that could turn his life around, or combine a certain anti-depressant with some therapy. IN the end he need's help, and he need's friends to help him with that.