I'm dating a couple, but one of them gets violent, and I don't know what to do

I recently joined an existing polyamorous relationship where I live in the home. They have problems, but what concerns me most is that at least twice now he has come close to physically assaulting her when he loses his temper. My question is, she says even if he hits her it's not my place to step in to help her. She says I'm thinking monogamously when I say if we are partners I'm not just gonna watch as she is assaulted. What should I do? Should I just watch or should I step in to defend her?

You need to leave that relationship and that home immediately. Violent tempers like that are not safe. If you are at the point of trying to figure out what to do about the threat of physical assault, you are at the point of needing to leave. Right now. Here are my resources on leaving a bad relationship.

Because your question was how to help someone in danger of assault, you should call a local crisis line. Google your area and “domestic violence” and you can talk to someone who can point you toward resources.

This has absolutely nothing to do with monogamy or polyamory. Healthy polyamory does not include trying to protect partners from each other, not does it include “just watching” as someone is threatened or harmed. She is operating under an bizarre and warped belief that their self-identification as polyamorous means that she can’t receive help from a third party who isn’t okay with her being the victim of domestic violence. It’s entirely possible that he is the source of this belief and is mis-using “polyamory” to convince her to accept his abuse and to keep her from getting help.

Ultimately, you cannot “save her” on your own power. Planning to “step in” during an incident of domestic violence is not a good plan. You can provide her with resources. You can state clearly and pointedly that his behavior is unacceptable. You can enlist other supports. Then, you need to focus on keeping yourself safe. Please get out of there.