In this case user has apparently given, some level of, consent to Cambridge Analytica. I don't know whether that would then make them a data controller in their own right or whether they still be treated as a data processor. If former then user would have to engage directly with CA for right to erasure or FB need to invoke T&C's. If later then it's down to FB T&Cs and then FB would have to inform CA of user invoking right to erasure.
GDPR will a minefield for consumers and organisations for at least another 2 years until we have some case law that backs it all up
However if hypothetically FB sold CA some data in 2015 without their users consent then it is CA that has the problem now as it has no consent to the data, not FB