Thats a tough one cause it would piss me off as well.
I would look at it this way,did he know it was your mothter?If not dont take it personal.Maybe he was having a bad day or maybe he is just an asshole which is probably the case.This is hard for me to say,just let it go no need for you to beat his ass and get busted.It would suck to loose all that money on a lawyer and court costs for a dickhead who has no respect for people just doing their job.I think your mom would feel guilty for telling you if you get in trouble and you dont want to do that im sure.So if you do wind up beating his ass wear a ski mask.