Fuck these pussies. Beat his ass then pour yourself a nice cold glass of eggnog w/Fireball and make that bitch show you her feet as payment for your troubles.
Your desire to help is commendable. You have a record, if you fuck him up you’ll be going away for a very long time. He’ll still keep doing it, and she’ll probably take his side and testify at your trial. She isn’t your blood relative who you have a duty to protect, your own family comes first. Unfortunately, you need to use the system. If you feel compelled to intervene, turn him into LA County Adult Protection Services. This is what they get paid for. https://wdacs.lacounty.gov/services/older-dependent-adult-services/adult-protective-services-aps/ They might not do anything ultimately, but it’ll be more than you can accomplish without fucking your own life forever.
Domestic violence is one of the most difficult and tricky situations trained law enforcement has to deal with. Don't try it as an amateur. It could go sideways in a hurry. If you witness the violence, then you can call 911 on the spot. If not, the most you should do is find the right number or web site and report your suspicions to the city or county.
Domestic violence in China and in chinese has been and is a huge problem. Changing that requires huge social change... Changing one guys bad behavior though is different... If he is part of a greater society that accepts this behavior, nothing you do short of violence will change him. If he is relatively isolated, though being chinese in California isn't exactly isolating, sometimes public humiliation works. In the end these abusers need to learn their behavior is not accepted... some learn by themselves, some learn by corporal punishment, some learn by public humiliation and some never learn.
Pass her the contact info when you next get the chance. She may not know that there are those who will help her.
Leave some domestic abuse pamphlets around the shop and then find a new place for your brew. “You Either Die A Hero, Or You Live Long Enough To See Yourself Become The Villain” Both the above options seem shitty to me.
Chinese culture has much to admire. Treatment of women, for some, has a long way to go. I understand wanting to take action, but odds are high that anything beyond what you have doe would make things worse. Do not assume the sone would take your side