My opinion - you are not responsible for your client's poor bookkeeping and should not band-aid a tax form to help them out.  Take the $5,000 into a Schedule C and based upon that prepare an extension.  If they complain, tell them it's their responsibility to give you proper information as you could be charged preparer penalties for an incorrect return.  If they wish to take the return to another preparer, fine.  If  lousy bookkeeping this year, it'll be lousy in the future and you may not want that type of client, and that risk.