That is so awful pipdae. Lucky for the frog you were there to see this and intervene but sad for the cory. I would have been horrified!
The frog most likely just saw movement and considered it food since they have such poor eyesight. ADF"S really do best in an ADF"S species tank only. They also have no tongues or teeth to even protect themselves from other fish that have teeth or are aggressive/territorial. ADF'S are very docile creatures.

How's the frog doing now?