The adult used crutches so was a recognised vulnerable person. Reporting the abuse is not interfering, no one vulnerable through age or illness should be subjected to public stress and humiliation by even their own child.
There is currently a murder case on where a teenage boy killed his stepmother . That behaviour doesn't occur in isolation, it is always part of a history of abuse of some sort. Who knows, someone might have witnessed bad behaviour between the two but never reported it?
The repeated bad language in public is also offensive to most. No one goes shopping in a store to hear a bad mouth. If the police heard it they would have interfered.
There are far too many vulnerable people being made utterly unhappy because they become the verbal punch bags of bullies. There is a clear definition of "vulnerable adults" online, it is not for the observer to decide if they think they are vulnerable.
We have boundaries defined by law and if we see abuse we must report it. It's not for us to "know the people", let the police do that.
In the last couple of years we have coersive abuse, where a family member/s hound and bully another family member almost to death. If the girl is disrespecting her mother because other family members are doing the same then the police can take action. Sometimes the only outward signs are a public spat.