I do not know CA laws, but would imagine no such law exists. There are boundary rules between a T and a client, but probably not about an ex... If you feel uncomfortable you really need to talk to the T about the situation. If you feel like it won't work out with being with that T since they are dating your ex, then I'm sure they can handle it professionally and refer you to another T, perhaps in the same place. But first and foremost, this needs to be addressed to your T if it is bothering you so much.

However, there are specific laws that prohibit your T to talk about you to the ex.