Why would a therapist want to be friends with former clients is beyond me. Huge red flag. Don't they have friends of their own? It's fine with emailing once in a while but that's not friendship, it's more like an acquaintance. As for whether it's legal or not, I would think the ethical part would be more important than the legal one. But apparently not (at least in the US). In my country friendships with former clients (and of course romantic relationships) are just a big no. Of course nobody is going to check but ethically therapists are not supposed to befriend their former clients. I've read several accounts of people befriending their therapist and in most cases it ends up being extremely disappointing for the client. And yet the topic keeps popping up on this forum.