The more I think about this the more that bit in ch. 48 seems like a mistake by the author. Harry did tell her. She presumably still has the "42" envelope containing the piece of paper where Harry wrote the secret obliquely. Ch. 48 reads like EY forgot that.

Edit: OK, technically Harry implied back then that she should avoid reading it unless she had to, so Hermione followed his wishes when she put the paper away unread. How do they expect this to work?

"That feels like a dementor. Better apparate away -- oops, can't do that, another wizard must have made an anti-apparation hex. Better pull out that wax-sealed paper I always carry and calmly open it with one hand, using the other to hold my wand in case I need it to defend against my postulated wizard opponent, and then calmly read Harry's hint while hoping the dementor doesn't make me forget who Harry is, and then figure out what he meant, and then apply the knowledge to the problem of duplicating a spell that Harry prepared carefully in a relatively safe situation."