I doubt that any of us even question clients about these lesser gifts. I've filed lots of gift tax returns but have never asked if they also gave their offspring $25 with a birthday card, bought them a sweater for Christmas, or took them out for lunch last Sunday. Nor has any client volunteered this info. Should I be asking? Would the IRS actually ask about this? The only ones I can think of who would care would be the states if the giver eventually applied for Medicaid.
Christian, Pub 550 contains detailed information and an example of how to report previously taxed interest. Essentially you report the amount on the 1099INT when the bond is cased (which will be all the interest the bond earned since purchased), the enter on another line "U.S. Savings Bond Interest Previously Reported” as a negative number. This will include the accrued interest her dad reported when he transferred the bond and the accrued interest she was required to report each year after that.
Since most savings bonds are online now, and this one will be when transferred, do advise your clients to create an inventory at treasurydirect.gov. You enter the bond purchase date, face amount, and serial number, and you can update it each year. While treasurydirect will give you the accrued interest each year, only the inventory gives you the lifetime amounts as well. This way when the bond is cashed you have a clear record of how much accrued interest was already taxed. (I once complained to treasurydirect that I couldn't find the total accrued interest, and they told me to use the inventory feature.)