This seems like an interesting philosophical question -- after all, the sender never agreed to let Google read her emails, right? But I think the resolution is that the receiver did agree to let Google read both emails she sends and emails she receives. Once the receiver "receives" the email, Google has permission to read it regardless of how the sender feels about it (if the sender didn't want them read, she shouldn't have given them to a receiver who would agree to this).

But Google wasn't using that argument apparently; rather they were claiming that the sender gave "implicit" consent -- not sure if this seems obviously true to me....