I worked from November 2011 as a free lancer with a friend but he recently passed away in December 2012. Now when I interview for a job I have a gap in my resume that I have no proof for as my friend cannot vouch for me. 

There is no invoice trail as my friend was paying me by cash. Interviewers don't believe I had a job at that time as I have no proof of working there.

How do I bridge this in an interview?