This is a bit similar to the thread on witnessing child abuse...
I saw a guy today going into and then inside a local shop, constantly yelling at his mother who was in a mobility scooter. She wasn't particularly elderly but obviously had physical disability, and potentially, from the way he spoke to her, mental health problems eg alzheimers or similar. But I was appalled at how he spoke to her. He was constantly yelling, "Come on mum", "get a move on" then actually whistled at her to get her to come over. He was also speaking to her in a really patronising way, "right come over here, don't mow the people down, just stay there and wait a minute," as if she were a naughty child (and I wouldn't even speak that way to a child).
I genuinely felt so sorry for this lady, and half thought about going over to speak to her to ask if she was okay or felt in danger from her son. I thought if that is how he speaks to her / treats her in public, what happens behind closed doors? But I was a bit scared to go and speak to her as he was quite intimidating. I've never seen them before, and obviously don't know anything about them in terms of who they are or where they live.
Don't think there's anything I can do but I felt very uncomfortable with the situation. Other people in the shop were also looking uneasy, averting their gaze etc.