I think it may matter which card game you're playing.
In the games that I play, you have to follow suit if you are able to do so. Otherwise, you are breaking the rules. So, I wouldn't expect the person to say anything. I would simply expect her to play her card. (In fact, in some games I play, talking about the game (called "talking across the table") while you're playing is against the rules. So the player *couldn't* say anything about the card she played.)
If I suspected they were or making a mistake (or cheating), I would tell her "you need to follow suit if you can." (or, in the case of some people I play with, "quit trying to cheat and play the diamond I know you're holding.")