Check this out, from the Preface of this document:
http://www.treasurydirect.gov/forms/sav0022.pdf "Required: Your financial institution is required to cash bonds/notes eligible for payment for a customer with the
proper identification. For purposes of this guide, a customer is any person presenting bonds at your financial
institution, whether or not they have an account."
When you go through the rest of the guide, there are instructions that clarify that total redemption value for this person who does not have an account with you must be less than or equal to $1,000, and s/he must have valid identification (from a list provided). Otherwise, you can instruct him/her to take the bond to his/her own financial institution.