Ok, sometimes these "scenario threads" disintegrate into silliness, and that's ok too.But there are usually some good answers and some thought provoking discussions mixed in. So I thought I'd go ahead and try to lay out a scenario. I'll try to keep it fairly simple and straightforward and see if we can agree on an appropriate response. If you normally carry concealed, then assume you're carrying your usual EDC. If you don't normally carry, then assume you're not armed with a gun.
You're at the local bowling alley with your significant other. It's evening, business is steady but not packed. There are entrances/exits in the front and rear of the building with parking on both sides. The rear parking lot is generally less populated and not particularly well lit. You leave your date at your bowling lane and walk over to the snack bar to get a couple of cokes. The snack bar is near the rear entrance. As you pass the back door, a person excitedly rushes in and says "There's a man beating a woman in the parking lot! She needs help right now!" Through the open door you can see a woman laying on her back in the middle of the parking lot with a male figure straddling her, standing over her yelling in an aggressive manner, but not physically striking her at that time. He is not visibly armed. She is clearly upset and crying. They are about 15-20 yards away.
Now what?
Call 911 and be a good witness.
She was there with some of her friends and they were bowling at a lane near the rear of the building. A person that I didn't know well but who knew me as a LEO ran up to me and told me that they just saw a man beating a woman in the back parking lot and that she needed help. I told the witness to call 911 immediately and to tell the dispatcher what was happening and also that I was there off duty and in plainclothes. 
