Lucy, we do know what went on in his house.
He subjected his gf to an assault which as other people have said, is actually a criminal matter. The victim would have been well within her rights to call the police and report this assault.
Thing is, I don't blame the OP for not recognising abuse. Like Lucy, most of us are socialised to minimise it and explain it away, particularly when it's carried out by people we like and have a good relationship with. I could never understand how it was my father had any friends, but of course he had them because he didn't behave in the violent entitled way with them, that he did with his own family - if he had, he wouldn't of course, have had any friends. He was nothing but charming, witty, helpful and kind to his friends and work colleagues. It was only his wife and children who saw the violent, vicious bully he was behind closed doors. Of course the OP thinks her friend is a nice, non-abusive man - which of us would voluntarily be friends with people we think are abusive? It's a huge shock to her to find out that her mate, who she's respected and esteemed all these years, is actually one of those men. Horrible and although obviously I agree that her response was wrong, I don't blame her for it and I feel sorry for her -she's also got a lot of coming to terms to do with the discovery that her friend is actually an abusive shit when she's never even suspected such a thing before.
Those of you who are minimising this because it's only a bit of water: this is how more serious domestic violence starts. It starts with an "accidental" shove, push etc., then moves on to harder pushes, deliberate man-handling, incidents like this, before it's the full fist in the face. What is important about this is not the water itself; it's the mind set behind it, the mindset of a man who doesn't have any respect for the woman he lives with and will use violence if he thinks he can get away with it, in order to exert power and control. That's why people are being so angry with you, because they know more about DV than you do and perhaps some of them have forgotten that we are all socialised not to recognise the clear signs of domestic abuse.
The gf has drawn a line and shown him he can't get away with it; please OP, don't undermine her, don't make her second guess herself; she'll be wondering if she's over-reacted, she'll be thinking about all the good times they had, wondering if his grovelling means he'll never do it again - she too, has swum in the same waters of denial and minimisation all of us have; she is very vulnerable to being told that she shouldn't walk out on this relationship. What is crystal clear to anyone with any knowledge of DV though, is that she absolutely should walk out on it, for her own sake. If she goes back to him after this, she is showing him that she will tolerate abuse. If you act as the minimiser and the friend who brings them back together, what you do is ensure that she knows that when he starts hitting her in earnest, she can't turn to you for support. Please don't be part of the problem curiousgeorgie, be part of the solution.