"Can I put that on her last 1040 or must I enter it on her 1041?"
Since the interest was not received until after her passing, it is income in respect of a decedent and is not included on her final 1040.
IRD should be reported on the 1041 if the estate received it, or reported by the beneficiary if the interest passed directly to the beneficiary or if the beneficiary otherwise obtained the right to receive the interest.
See https://www.irs.gov/publications/p559#en_US_2017_publink100099582
