Name changed because I'll give details.
City centre, romantic restaurant it's a small place with only around 6 tables. I am in the window area at a 2 seater table with one other two seater right next to us.
Me and partner joke around a bit order food and relatively happy. I noticed he's stopped making conversation and so him if he's okay. He sends me a text descreetly saying the man on the table next to us is being abusive to his wife/partner.
I try to listen but only catch bits. He puts her down about a new job which she was excited about. Tells her she isn't ready and can't do it. He's slurring and a bit drunk. She calmly asks him if he's jealous and he says 'you can put that fucking idea to bed right now'. I can't hear well and sort of try to go back to what we are talking about but then the man throws a huge tantrum and starts refusing to pay. She ends up paying. He makes more rude comments. She politely thanks the waitress, she looks pissed off. He refuses to move. She says 'shall we go then' after a few more minutes and he just sits there, he says 'stop telling me what to do'.
She ends up going to the bathroom and my partner says maybe I should follow and ask if she's okay but the restaurant is so small it only has a single cubicle toilet so not an option. When she comes back after a few minutes he still doesn't want to move. Eventually he seems to deem this round of punishment over and allows them to leave. My partner tells me more stuff he's overheard him saying, all in a similar ilk.
I was quite shocked at this behaviour in public and my stomach turned at the thought of her having to go home with the nasty old prick. I guess this more a wwyd? Would you have said something? If so what? I'm usually quite outspoken but it didn't feel like my place and I didn't want to cause a scene in the restaurant, or make him angrier and it makes things much harder for her or even dangerous as she's got to back with him.