I know this may sound stupid, or lazy, but how do I transfer the signed 8879 from my email to the client's return?

[itonewbie · Accepted solution]

You don't need to.  It just needs to be saved on your file for 3 years since it doesn't need to be filed with the return.

Because you received the signed copy via email, my presumption is that your client must have received the return along with the F.8879 (via encrypted email).