1) Traiger & Hinckley, a law firm that specializes in CRA compliance, published a study of high cost loans in January of 2008. They found that close to 85% of all high cost loans originated from non-CRA banks.
Link to the study here:
http://www.traigerlaw.com/publications/addendum_to_traiger_hinckley_llp_cra_foreclosure_study_1-14-08.pdf#page=22) As Fannie and Freddie are CRA-compliant, they had little effect on the mortgage securities crisis. They did relax their lending standards and did act irresponsibly, primarily out of a necessity to compete with private lenders who were not subject to the same rules, but they were no where near the heart of the problem in terms of the volume of mortgage securities being bought and sold.
There are other problems with the argument as well, but if you're talking to people who have no background knowledge on how securitized loans work, you'd most likely be talking over their head. The bottom line is that the low volume of high risk loans being traded by Fannie and Freddie, when compared to the private sector, doesn't give the argument any credence.