He Knows Her Secret
Josh already knows Liza is lying about her age. And, yeah, it took him a while to come around, but he eventually did. What's more, he's willing to help her keep the secret. It's the kind of support and understanding Liza needs while she figures everything out.
Charles, on the other hand, doesn't know her secret. In fact, Liza can't afford to tell him as long as she works for him. It's a dangerous relationship to get into. And who knows how he'll react if and when he finds out?!