Glad to provide a happy update. He ended up with just a warning because of some mitigating circumstances.
The assignment was the same -- essentially, describe how assigned reading [x] makes you feel -- and he felt essentially the same way, as might be expected. Somewhere between "very little" and "around half" of the assignment was copied over, depending on if you count using the same quotes from the assigned reading.
Because of the similarities, the prof wanted to ask follow-up questions about his new response, so the prof emailed BIL (online class; can't talk after class). Problem is, the prof emailed his UT email address when both the UT student directory and the prof's "fill out this contact/background info" sheet at the beginning of the semester had BIL's gmail address. So the professor emailed the wrong address in follow-up, got frustrated after feeling he was being ignored and turned him in.
When the hearing came, BIL pointed out his GPA, pointed out that he never had any history of academic misconduct back through high school, and that he had other profs lined up to serve as a reference for him. He also offered to re-do the assignment with the caveat that he would change the form, but the substance would be similar because he truly feels the same way about the assigned reading.
The prof tried to flex by asking BIL why he ignored emails from his prof, including as recently as just a few weeks prior.
BIL said he received no such emails and that he would happily show his gmail inbox, spam, and deleted items folders to the committee.
Prof tried to shazam him by showing printouts of the emails from prof to BIL's UT address.
BIL went into [Chalkboard/blackboard/whatever learning management program UT uses] and pulled up the "assignment" that was the prof's own contact info sheet, as well as the UT student directory, both of which showed that the gmail address was how to get ahold of him.
There was a collective "ooh, so that's what happened..." from the hearing people and they gave him the absolute minimum allowed under the applicable rules, once something has reached the hearing phase. Not sure if the "warning" was verbal or written and didn't follow up with my BIL or my wife.
At least that's the story as was related to me.
So he ended up with a warning with no adverse effect on his grade for the semester and no requirement to re-do the assignment or otherwise do anything else.
THanks again to everyone for your thoughts and help.