Several places to check if you think the guy had a Trust:
3. Safe / Gun Safe
4. Safe deposit Box - look at his bank statements - then call his bank and inquire.
If you have access to his computer (if he had a computer) you might find an electronic version of the trust. Also, look for any correspondence with a lawyer - might give you some clues.
Most folks I know that do Trusts just to acquire an NFA item keep at least a copy of the trust with the NFA paperwork very close to where the NFA item is generally stored. These are just some of my thoughts - Sorry about your friend - you have your work cut out for you.