I agree sf618 that relationships are not all rainbows and butterflies and arguments do happen. I agree that you need to set boundaries for how you argue and STICK to them. If either one of you crosses the line, I think it’s a good sign if there is genuine remorse which includes action steps to ensure that it doesn’t happen again, not just saying the words.
That said, there is a difference between someone who does something which they bitterly regret in the heat of the moment (like sf618) and which is totally out of character for them, and someone who has a pattern of behaviour that is abusive. I would really encourage your friend to get to the bottom of which one it is.
* How long have they been together?
* Is he at all abusive in other ways, emotionally abusive, belittling, controlling etc.?
* Is this the first time he has ever been physical with her in an agressive way? Sometimes these acts of agression can seem fairly minor, like pushing past you or grabbing your arm, or breaking a plate, but it’s all part of the same family of behaviours.
* Does he have a temper?
I would also really encourage her to look at the reason why she dug her nails into his arm. How does she feel in their relationship as a whole? I’m not excusing her digging her nails into his arm, but if he is controlling or manipulative or abusive in some way, that can lead to a huge build up of anger and resentment in a person.
My point is, it’s fine to set boundaries, but she needs to get to the bottom of why they both behaved this way. Being physically abusive with someone is very explosive, and it means there are serious issues in the relationship that are not being addressed. It’s not right what she did, but as I say, it could be that she was provoked to dig her nails into him by a huge build up of anger inside herself. She must get to the bottom of why she felt she had to do that, and that might mean work on herself.
As far as his behaviour is concerned… grabbing her by the throat is totally unacceptable. If it were me, I would walk away from this relationship immediately. But I would definitely not recommend that she stays unless he agrees to speak to a therapist and get help on an ongoing basis.
The problem with physical violence is that it tends to escalate, and that is why it is very important to determine whether he momentarily did something which he really regrets and which was out of character, or whether it is a pattern. I will add that every abuser is very apologetic after the first time and wants things to go back to normal as quickly as possible, but it is very important that he doesn’t pressure her and gives her the space she needs.