There’s two issues here. The first is the Evil god pretending to be a Good god. That’s mostly been handled pretty well by the discussion so far.

The second issue is the Evil god pretending to be THIS GUY’S specific god. If the Paladin was actually devoted to a specific deity before (instead of just generically LG, which is totally possible but far from guaranteed), I don’t see much of an elegant way for the Evil god to pretend to be whoever he was devoted to before, unless you’ve got some convincing reason why this Evil deity would directly interfere with a mortal but that mortal’s sworn divine patron (who is probably at least nominally invested in their own capital-P Paladins) wouldn’t get involved somehow. If you’ve made the gods be distant beings who pretty much don’t ever interfere directly in their followers’ lives (even the members of “divine classes”), then maybe this is just a powerful but non-deific fiend who’s a little bit more involved in flesh-and-blood matters than an actual god would be, but otherwise, I can see a player calling BS on their actual god not stepping in somehow.