i feel like 2 things can happen here:
> he will tell her about the book but that wont be the problem, the problem is gonna be he didnt read it (probably has a note,annotations or something) and she will get mad
> he doesnt tell her and replaces the book and after reading it he just returns it and she's like: are we not gonna talk about ''the thing''? and he will go the what? then she tells him aboit it and he will have to come clean about fucking up the old book and then the problems arise