DV laws in Oregon

[MORA Ranger]

I have a question about domestic violence for any of you that may be farmiliar with Oregon laws. I know in Alaska, as in a lot of states, there is a mandatory arrest of the primary physical aggressor in DV situations. In Alaska, those situations are defined as being between people who live or have lived with each other. No sexual relationship is necessary. In AK if I were to get in a fight with my roommate it would be a DV.
So my question is about a very good LE friend of mine who was visiting her family in Oregon. She's never had a good relationship with her dad, who sounds like a lot of people I've arrested. They got in an argument and he got in her face. He was jabbing his finger in her face and she said, "What are you gonna do, hit me?" or something along those lines. So he did. She's 23 and she's been a cop for a year. She was worried about loosing her job so she didn't hit him back, instead she called 911 and then hung up. Then her mom joined the fray and got between the two of them and her dad hit her mom. My friend started wrestling with her dad and put him in a head lock. Then the dispatcher called back, the cops showed up, did their thing, and then left. They didn't arrest anyone, nor did they contact the DA (which is what I'd do in AK if we couldn't decide who to arrest).
So, after this lengthy story, my question is, why wasn't her dad arrested? Doesn't Oregon have mandatory arrest laws? My only thought is that they must have a different definition for the types of relationships that are considered under any mandatory arrest requirements they may have. My guess is they looked at the situation and called it mutual combat. It must not have qualified as a DV for some reason. What are everyone's thoughts on this? What would you have done? Need more info? I don't think there were any visible marks beyond a red mark on my friends face and on her mom's face. Nothing on the dad. The officers didn't require anyone to leave for the night or anything.
I would've arrested the dad, but I know that he's been beating on his whole family since my friend was a kid, though she told me he's never been to jail.
Thanks for any input!

[darrell]

I don't think the laws says "Thou must arrest." But more so that "thou shall arrest." If you understand what I am saying. Here in Michigan our county is a must arrest county where the county to the north is a shall arrest leaving discretion in the hands of the officer.

[Lfpdlieu302]

we dont have a must arrest policy here. it is primarily left to the officer on scene. however , with that in mind, an arrest is prefered in order to get no contact orders and such.

[sbrad]

Ours is a preferred arrest state.
If there were red marks on both of them and both of their stories matched, then I'd hook him up. My question, though, is what did the mom say when they got there?

[MORA Ranger]

The mom told the officers what happened. Oh, I forgot. When her mom got in between them, the mom slapped the dad and then he hit her. The mom basically just said that the dad hit my friend, she called 911, and only went after her dad after he hit the mom. Confusing enough? Thanks to those of you who have responded, I didn't realize there were places without mandatory arrest laws for all DVs.

[txinvestigator1]

This is really an impossible question without speaking to the officers or knowing the situation better than 4th hand