> I would guess his lawyer didn’t really believe him

Then you'd ask to change lawyer. If they can apply a standard practice of care and do basic checks like matching a username and time of use because they don't "believe" you when you say these are not you messages, they are not doing their job as a lawyer.

I'd go as far as saying such basic checks not being done amount to negligence/malpractice, they might have a civil case against that lawyer.

But the lawyer would say "okay, help me prove you didn't send these messages"

We know he's innocent, and he knows he's innocent, but how does he prove it?