From the sounds of it, the OP was asked and granted permission for her therapist to speak with her mother; it wasn't a requirement. Since this is something they discussed and the OP chose to have this happen, it isn't particularly a boundary crossing as far as I can tell. All that said, in this particular situation, if I was the OP I wouldn't want my parents speaking to my therapist if I wasn't present simply because it sounds like this particular parent is rather unreliable to say the least. It is unclear whether the meeting outside the OP's presence is okay with her or not. I would hope that was a discussion the OP and her therapist had and that this arrangement was one she was okay with.

A therapist speaking with parents, even of an adult client, isn't necessarily a bad thing. Not all parent relationships are dysfunctional, and family support can be extremely helpful. I just am not sure at all that in this case this particular parent can be trusted to be honest, nor does it seem at all that the parent is a healthy support to the OP. The therapist doesn't seem to be taking that into account in this situation, and that feels all sorts of uncomfortable.