I would suggest that OP brings it up again and see if they can email the restaurant's owner. They could easily not bring up the specific name, but rather say they had an \[unnamed\] waiter who laughed at her stutter. This could also mean they educate all of their staff about this and not just the one employee. She might not feel comfortable with this or frustrated she has to be the one educating but this might be an easy way to do it. I'm a college student and an employee at a dining hall constantly finishes my words, I might just email them so their staff can be educated. edit- If she doesn't want to, it's okay to not do it. Or if she looks uncomfortable, possibly bring this idea up the next time that you (possibly) see someone make fun of her.