> 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.