Um.

It doesn't really matter who you think was on the other end of the conversation. Niece, neighbor, undercover cop, they're all irrelevant.

What matters is your husband's end of the conversation. Apparently, your husband's end of the conversation was more than enough to charge him and land a state for the case so tight that his options are trial or plea bargain. Instant messenger conversations are stored on the provider's servers, regardless of whether they're archived on your local machine.

Your husband can certainly ask to see the evidence against him. His attorney is under no obligation to share it with you, however, and while you might think you can tell who was on the other end from a transcript, there's nothing you could do with the perceived information in any case.