I would agree with the aphids statement. Ants eat their eggs because they are sweet and sticky to them.
BUT, with aphids, she would have seen them and I dont think aphids would have survived just on the top of the corn. They usually go for the bottom sides of the leaves, or in the corners near the stalk.
It would also take some time for them to reproduce enough to attract that many ants. She would have seen that. ( I think ..)