I would not draw my firearm in this scenario. You did not mention any instance of "deadly force" in the situation. Yeah the guy grabbed her, yeah, she cried for help, but I did not read anywhere that the guy had a knife, club, gun, etc. It's not a very good situation to be in. You already have a friend calling 911, which is a good thing. Next move would be to seperate the lady from the alleged perp. Notice I used the word "alleged". The reason I used the term is that you don't know what went on prior to the lady running outside and screaming for help. It could very well be that the guy found out she was messing around on him and threatened to leave. At that point, maybe the lady assaulted the man possibly kicking him someplace sensitive

or maybe even using some type of deadly instrument (like a firearm or knife). It's possible that the guy disarmed her and she attempted to get away by running out the door yelling for help. I'm in no way condoning domestic violence. I'm putting the "what if's" out there so others would think before acting. As a firearms instructor, it's my duty to educate my students on the legalities of using their firearms. Part of this would involve talking through scenarios as you described. My advice is NEVER draw your firearm in defense of others unless you're SURE (be it "positively", "absolutely", 99%, etc) of what the situation is AND you are willing to face the consequences of your actions. You will most likey be arrested, or at minimum detained. You may spend some time in jail. You most likely will spend a lot of money on legal fees (contrary to popular belief, I woudn't bother with services like "Prepaid Legal". Think about it, if you're a working professional earning a certain rate of pay for your services, would you really want to be making less money per hour and still put out your 100%? I'm not slamming the service, just speaking from persona experience.)
Once you get the two parties isolated and everyone is "safe", wait for the police. It's their job to investigate and question both parties to figure out what happened. You may want to have your photo ID AND carry permit ready to present to arriving officers. Domestic violence calls are HUGE wild card for LEO. They never know what knd of mess they're walking into. It could be a simple argument between two parties, or it could be a large family duking it out. They don't know if anyone is armed or any other potential dangers. It's best to let the LEO do their jobs when they arrive and cooperate the best way you can. By being a good witness, you could provide valuable information to LEO.
As for physically restrining anyone, DON'T DO IT! We've discussed the matter on other threads. Anytime an ordinary citizen attempts to "arrest" another citizen, many bad things can happen. If you are not properly trained in using restraints, you could cause serious injury to yourself or someone fighting you from being restrained. Then there's the legal aspects of a "citizen's arrest". Unless you're in the right, there may be criminal and/or civil violations on your part. This may result in monetary loss as well as encarceration on your part. Leave the arresting to LEO.
Hope I've answered your question. Keep in mind that
I AM NOT A LAWYER. THIS IS NOT LEGAL ADVICE AND SHOULD NOT BE CONSTRUED TO BE LEGAL ADVICE. IF YOU WANT LEGAL ADVICE, CONSULT A COMPETENT ATTORNEY IN YOUR AREA.
gf