What do I do when witnessing possible DV?

TW for DV

I hope this question is allowed. It's less of what do I do and more of a what should I have done/do if there's a next time?

I was recently waiting outside a large supermarket with my son for my husband to collect us. A couple came out of the supermarket, she carrying a young child, approx 1yo. He was shouting, swearing at her, accusing her of lying, effing and jeffing, aggressively pointing a finger in her face. She was visibly upset, red in the face, teary. He was accusing her of lying, of going in the toilets to text her friend. He was just so over the top verbally aggressive and it was awful. Really shocking.

I wanted to help her but didn't know what to do for the best and didn't want to make it worse or provoke him into physical violence towards her or me. I saw a shop employee a short distance away, stacking trolleys and I went to ask if there was a security guard in store that might be able to help the woman. He could hear and see it all going on as well but he just... Didn't seem to care? He said "Well, there is but they are agency so I don't know if they will do much." I asked if he wanted to go and find out because the woman was clearly in quite a bit of distress. Meanwhile the poor woman had been buckling her child into a car seat and her partner hadn't let up swearing, shouting and raging at her. By this point, she had begun to drive away and he had his finger in her face, swearing at her while she drove out of the car park.

I just couldn't stop thinking about her and the danger I worried she might be in. If he was willing to be like that in public, what was going to happen to her when they got home? Later I thought I should have taken a photo of the reg plate and phoned police for a wellness check and kicked myself for not thinking of that sooner.

I just wanted to know if anyone had any advice for if I were ever to witness something like that again? It felt outrageous not to help but at the same time, I felt quite vulnerable myself, there with my son. I was a coward, I think and I wish I could have helped. At the same time, I really was worried about inflaming things for the poor woman.

Please be kind in replies.