What your wife is doing sounds unhealthy and possibly even dangerous. I don't blame you for feeling concerned.

Suppose you were to totally ignore this. If she is talking about this celebrity, suppose you were to always change the subject. I'm suggesting that you withdraw any and all attention on your part to Zac Efron and your wife's claimed interaction with him. You would withdraw some energy from this obsession. It might then start to wither.

If your wife is going to entertain this delusion, it may be impossible to reason her out of it. Challenging her may just cause her to dig in her heels, insisting this is a real connection. So maybe try doing the opposite. Show zero interest in anything she says about this celebrity. Try it and see.