There has never been a "signed" copy of the return ever.  The only signed copy is with the IRS.  Taxpayers have always provided their copies as supporting document.  The signed F.8879 and E-File Confirmation Letter should be proof enough, if necessary.  Otherwise, your client can request a transcript from the IRS.

If you are in the midst of these discussions between your client and the lender, hope you are getting paid and have the proper authorization to speak or furnish tax info to the lender as a third party.

In our engagement letter, we clearly states that the returns we prepare are only for filing with the relevant tax authorities and not intended to be used for other purposes.  Clients are free to use the returns as they see fit but we won't get in the middle of other financial transactions they engage in.