I actually mulled that myself, but that would mean Billy and Mary's dad is a) also evil, which would be dumb and annoying; or b) a cosmic dupe, somehow convinced that he's in service to higher powers as penance when he's rather the opposite or c) an impostor of some kind, either of which seems too obscure to merit randomly bringing the character in out of left field like that as a literary choice.
I mean, it'd make the use of Jay make more sense - if a good guy imprisoned an evil Shazam such that only another good guy could get to him to free him, a dimension-hopping stalwart like Jay is a good place to go to solve that. But what meaning does Billy's father have for Jay? Seems like an odd choice of messenger.