As a person I like to think that if it was my son I could look at your situation and say "You had to choose between a child you KNOW was in trouble and ones that you trusted that were NOT. You made the right choice in going to help the hurting child." But, I am human and a very protective parent and if it was my kid involved I can't swear that I'd be that logical about it... at least not on first hearing about it.
Down here if there's a scuffle BOTH parties are punished. To the point that once my sister and brother were arguing VERBALLY in the front yard and the cranky neighbor called the cops (mind you this is like 4pm, not 4am so it wasn't a noise violation or anything) and told them the were 'fighting'. The cops arrived and ticketed them BOTH for fighting. Quite a large fine actually. It didn't matter that they hadn't exchanged blows, that friends in the yard said as much, that physically neither was in any way scuffed up. All that mattered was that one crotchety woman SAID they were "fighting" so they assumed violence and punished accordingly. If that girl gets cited for that it will follow her. It will effect her college applications, acceptance, scholarships, future job applications, and so on. Right now I doubt his parents are thinking about that, maybe they never would, some are horribly hateful that way. If the dad is half as macho as he sounds then odds are he wants her punished for 'unmanning' his son more than anything. If he instigated this girl, then he deserved what he got, IMHO. If you truly feel he is lying, then be 100% honest about your suspicions. Tell the TRUTH. Cite examples of his fibbing in the past and let those in charge take it from there. That's really all you can do. Oh, and I'd keep an eye on that little bugger too. In my experience there isn't much worse than a sawed off little jerk who's looking to prove his manhood. OY VEY they can be real terrors. Most of them grow out of it, but some do not. Some, if not corrected, only escalate the behavior as they grow older. For that reason alone I'd be keeping an eye. Phew that ended up a ramble, sorry. You sound like a good teacher with a good heart and a strong conscience, you'll know what you need to do. *hug*