Wait for it to happen again (in person, on phone, via text or IM) and make it short and sweet, be kind and firm: W: this is not kid or money talk and I don't care to have this conversation or any other one that is not related to the kids/money.
If she asks why - say you are honoring a boundary SHE set as that is the healthy and productive way to improve communication then let it go.
Each time she tries to sneak in another topic say the same thing. Eventually she will get it or ASK you if you if she can change the boundary.
