I wrote a simple two sentence letter and mailed it. I did not provide any explanation. I know that others have written and posted exemplary disassociation letters because they are opportunities to highlight iniquity. I had 'responsibilities' and felt that I needed to give them notice that I was done, finito. I also wanted the clean break and to be left alone, free in my mind to get on with my life. But your brother's friend needn't have asked because if you had DA'd yourself formally the news would have spread FAST.