Wwyd? Witnessed abusive, bullying parent

[Blep]

I was on the bus and a woman and her teenaged child got on. We were the only people on it. They sat apart from each other. They'd clearly fallen out over something, and she was saying how she was going to punish them by taking their phone away etc. Child answered back, and this resulted in them shouting childish things at each other back and forth.

There were periods of silence which were broken by the mother goading the child, followed by more arguing and shouting. At one point the mother threatened to smack child in the face (verbally, they were nowhere near each other).

I didn't do or say anything and I feel awful. What should i have done?

Nothing. Lots of people are going to interact in a way you find unkind and unsavoury. It sounds unpleasant but it happens. Who would stop it and make them speak nicely?

I don't know. Perhaps you did the only thing you could by staying out of it. You're not going to change her mum's behaviour with one comment, you may well have just made her angrier.