I think the loan officer is wrong. Form 8879 is used to authorize a tax preparer to either use your pin (which is supposed to be secret) or to generate a PIN for you.
I think what will probably make the loan officer happier is if you print a copy of your tax return and sign it yourself in ink. If you need proof that you also filed this return, there should be a page marked "electronic postmark" in your package if you download or print "all forms and worksheets."
The form is here if you want it https://www.irs.gov/pub/irs-pdf/f8879.pdf