Wow that's a toughy. I'd be questioning my mom about how he said it and such. If he knew it was your mom- it's a straight up get his ass. If he did'nt know and being a bully on purpose- I'd say: hey - can I talk to you a minute?
Then I'd just be straight with his ass: Hey you said some messed up shit to my mom in the bank the other day man that made her feel bullied or whatever it made her feel and that's got me a lil pissed dude- put it on him - be straight and let him take it how he wants it- fuck all that game playin or ski mask shit- be honest and if he wants to take it there - then go at it face to face like fuckin men. Just dont kill him- lol And especially dont get your ass beat. But be a man about it- if he wants to be a punk- treat him like one.
Jail- consequences- not to be feared more than doing the right thing. Be smart but not sneaky or cowardice. If ya'll fight as long as you dont use a weapon or kill him or whatever and no witness says you started it- the charge wont be that bad.
If he did'nt know and she aint too upset I'd still pull him to the side and just be more friendly about it. Otherwise I'd look him dead in his eyes while I was talking to him serious but not agressive.