I have a client who wants this web-based membership system I'm building to also accommodate arbitrary changes she makes to the user data in a third-party system that my component writes TO but doesn't receive data FROM.

No amount of explaining dissuades her from the belief that her changes will magically be represented in my side of the data.

So, yeah, I'm screwed.

@jAsE education done. She either refuses to understand or doesn't care. I'm seeking an exit strategy on this project anyway (firing the client) once it launches.

@jAsE it's a delicate thing sometimes because not all client/provider relationships that need to be terminated are all that bad, except for scope creep on the project that makes it so unprofitable that you just can't work for free anymore. You have to be very diplomatic to avoid damaging your brand or give them an excuse to trash your brand in public. Make it feel like the only good solution for both parties is for the client to seek help elsewhere. Others are easy because their non-communication turns into termination by default.