Suggest Treatment For Severe Depression And Obssesive Nature

Question: My brother has been in a relationship with a married woman for 4 years. They broke up a couple of times for very short period. In February she advised him in a very brief and abrupt phone call that she had confessed to her husband and was ending the affair. They never went out in public, would correspond by email or she would go to his apartment. My brother cannot seem to move on and in fact seems to be obsessing about her. It's been 8 months and he seems very depressed, upset and won't get over her. At first I tried to listen and sympathize with him; however, he only hears what he wants to hear. He has tried to phone her, email her, had his friends try to contact her. All he says is that he needs closure. However, he finally went so far as to contact her minister. This resulted in a 4 page letter from the woman indicating she was in love with her husband, was working on her marriage and wished my brother happiness and hoped he would move on. Of course, now that isn't closure - or at least what he wanted.
My brother and this woman only saw each other at their very best; he holds this woman this woman up on a pedestal and sees her through XXXXXXX tinted glasses. To me it seemed the woman was not satisfied with what she was getting from her marriage and seemed to come to my brother when it was convenient for her and when she wanted attention. He's now crossed yet another line and contacted her minister who indicated she was completely uncomfortable about that. It resulted in yet another letter hoping he would move on, etc etc. My brother accepts no part in intruding in this marriage; indicates the woman is in no way to blame for having an affair and everything is the husband's fault. It's almost as if my brother feels like the husband/injured party. I finally tried to explain to him that all these things could be perceived as harassment/stalking by the police and he could be jeopardizing his reputation and career. He couldn't even comprehend that someone might think that. At this point he hears only what he wants to hear; reads between the lines and sees what he wants to see and no one can get through to him. I'm extremely worried about him. What more can I do?