I think T's can be friends after a year or so after (I know T's are supposed to wait that long to have romantic relationships with former clients). There's T's that collaborate writing books with former clients, etc. I think the "rule" to not be friends has to do with protecting each other since the therapy relationship is "special" with special rules and not like a friendship. After a bit when things have changed to "normal" in the client's life then T's and clients could be friends I suppose but it would have to be mutual? T's have their life and friends just like we have our life and friends and there may not be much in common or time/interest on both people's part. Usually the T relationship is about the therapy and once that is resolved/through, there may not be anything else in common.