Wow - lot's to think about here. Interesting on "What if..." level. What would I do for such a person? Hard to give my best answer....lot's of assumptions. Let me see if I can hit the high points.
#1 CAREFULLY consider even getting involved in the first place. As was mentioned by many - the frame of mind / wellness of the person in question is a HIGH concern. You have no idea how/why this woman is involved with this guy. But let's go here...
Best Case Scenario: She was completely wronged by the accused. He is crazy. If it gets to him that you are assisting in her protection, you and yours are now in his sights potentially.
Worst Case: She is a damaged person who will forever be beaten and abused by those involved. You are wasting your time.
Strange Case: She is now psycho and you will be implicated for her plotted murder of the guy wether he is found guilty or not. Can you get hung for this? I doubt it, but I don't know. Can it be a hassle? Oh yes.
Domestic violence happens for a reason - Not the first reason, but the biggest is that women allow it. Not saying they can't change. But I have sat in that court room on a jury watching police time and tax payer dollars get flushed down the drain...
#2 Counseling, legal dotting of i's and crossing of t's, and layers of protection. One of the best threads here. That advice ALWAYS applies. Way better than the details of this vs that firearm. Move / hide / change the numbers, restraining orders, alarm systems, dog, good doors/locks/windows etc...go very very far.
Assuming that she is correct in her presentation (and I don't know - so I'm devil's advocating), she should be doing ALL of that as well. Better for him to not get in than get in and the shoot decision need made in the first place. Of course, if she invites him in to discuss things...all for nothing. By the way - new locks are expensive. If she hasn't prioritized these things and the money for them - she needs to get her head right.
#3 Which gun? I have said this a ton, and I'll say it again. For home defense:
The number one thing you need is a PLAN. More important than any specific weapon.
The plan should ideally be: Sit in safe room with 12g (or 20g...) and a cell phone pointed at the only entrance/choke point and wait for the po-po.
I honestly don't care what the gun is. This is more important than any of it. Once the use of the gun comes into play - all kinds of facors at work. Let's avoid it and stay safe before it even gets to that. But if it gets there - lt's keep us, the neighbors and everyone else involved/informed/safe!
#3 If we go gun, and we do have a plan, etc... #1 issue is willing to get the skill. The cost of a gun - let's not even worry about the semi v wheel factor is nothing. The number one way to skill and competency is time on trigger. Time on trigger requires a range and ammo. I'd generally say a cheaper gun and more ammo is better than an expensive gun and no ammo. But how many rounds to you think are needed for basic self defense competency? I'll throw out 2000 (low # imo). In 9mm WWB from wally-world, that's roughly $500 - not including the range time if needed, or the gun. Or the time to shoot!
As was said about mindset, she claims this, but as always, $$$ comes into play. As a prepper, I always say better to have something cheap and now, than a plan to buy something expensive when you need it. But you can't discount that to use the Nasal Pharyngeal Tube you need training...and it's pretty useless without it.
If she isn't going to train, better to invest in the locks etc, IMO. But there is a cost to safety. Check out what it REALLY is.
I always do this drill at the range; The VERY FIRST SHOTS of ANY trip: Wife (or me) closes eyes, puts gun in hand, and I put the target somewhere closer/farther. Tap on the shoulder and open and shoot. Would you have STOPPED the threat? For fun, add a snap cap, hand the gun is less than good to go, or mag out etc...and run the clock. It will make you think...
Training basic/active shooting is a second great idea. But again, $$$.
#5 OK - we are all the way here. Which gun?
I still say, if we are in Home defense mode, dollar for dollar, effectiveness overall, amount of practice to competency, the all around winner will be the Shotgun. Less expensive gun, long gun for accuracy benefit, shot factor for slightly larger window on effectiveness, easier to gain competency in less rounds of practice, than ANY handgun.
If we are going handgun - I honestly don't care. But like I said - see the "skill" factor above. I can put a 12/20g in someone's hands, run my little range drill and get them extremely effective in hundreds of rounds - not thousands. The complexity factor of a semi is a factor. The barrel length/trigger/grip on short revolvers is an even bigger factor.
I'd say go semi in general. The reliability factor is ONLY an issue if you haven't actually tested the reliability. Old timers need to let go of that. My USP has NEVER failed. It will put more lead down range, faster, on target than ANY revolver you can put against it. I know every mag works perfectly. I know the gun works flawlessly. But I have trained with it extensively. Including tap/rack/bang, mag changes, etc...
Revolvers also have problems - they CAN fail. When they do, tap/rack/bang doesn't make them go again. My main issue with a revolvers (and don't get me wrong - love them!) *assuming short barrel...
#1 Short barrel - practice can overcome anything, but a nice 4-5" barrel on a semi is a good thing when it comes to hitting a target.
#2 Trigger - tough triggers mess up accuracy period. Hard to beat a semi in SA mode.
#3 Capacity - hopefully you won't need it, but you can't use it if you don't have it.
#4 Sights - see number one - especially if short barreled.
Yes practice can overcome any of these. But again, you need to practice. A DAO, 5 shot, short barrel.
I know a lot of this is assuming that carry is not the issue, and snub is the choice...
Still a lot of issues before the specific gun even comes into play....
Intersting situation Good luck. My sister in law (who is of questionable judgement) calls people who she has been to their house once to deliver a party invitation "friends" if they have a nice foyer. Just a final thought.
For the record - as a husband and father of a daughter: If this dude behaved as it was stated...I think death penalty is appropriate much more so than jail. Nothing burns me more than my $$$ paying to feed human waste and scum like that.
Good luck.