First - they are very valuable. The bonds accrued interest for 40 years, gaining value to 3.6 times face value (or close). That means the bonds are worth almost $18,000.
Second - you have to prove that you are the rightful owner of these bonds. Ouch. I don't think a garage sale receipt is going to do it. The bonds really need to be redeemed by the person who bought them or a rightful heir. I think your friend probably needs to do the right thing here and call the person he bought them from and explain that he bought some very valuable things at the garage sale but they still belong to the rightful heir (or maybe the person who bought them). He ought to insist on getting a really nice home-cooked meal at least out of this.