Duchess, if he's been arrested that many times for assault, it's highly likely he's currently on probation.
If I were you, I'd go to the probation office (or criminal court house) of the most recent arrest, and take his name and birthdate (I'm sure it's on one of the mugshots) and ask for the resolution of the case. i think it's public information. You may have to pay a small search fee.
At this point, he's really escalating further. Had he come to the game kind of angry, this wouldn't be as big a concern. As it is, now he's devised a grand plan, which is a great escalation from lamely calling someone and leaving a threatening message. Hope your friend kept the message, though.
At this point, this guy has a focus - to bring down this coach, and he's putting a great deal of energy into a fairly long term goal.
You need to know who his probation officer is, and let him know if this doesn't stop NOW.
Best wishes. What a complete loose cannon.