Whether you stay in this relationship will come down to if you perceive the benefits outweigh the problems. I have friends in similar situations that chose to remain and just deal with it. (see below for a recommendation on dealing from your end)
The problem for most of them is the unknown after the spilt. Dating and finding a suitable future mate. It could end up miraculously great, but the reality is that there just aren't that many 40ish single women out there who are great people, nice looking, in good health, with good incomes and spending histories that are also not on the psycho spectrum. If you want an independent one, finding one that would also want you might be very rare indeed. Most of the 40ish women will also come with baggage of kids and divorces. One has to ask if a woman is 40ish, has no kids, never been married and has all these great attributes, why that just doesn't add up?
Probably only 7% of the female population falls into your desired age category. Of that maybe 80% are in relationships. So you are down to around one in 100-200 being available. Of those maybe 10% meet the above criteria. So you're looking at maybe 1 in a thousand. Add that she also has to be into you and the odds get very close to zero. (sorry to be such a downer here)
Regarding the physical abuse --- I don't see that as a big a deal as the psychological abuse. Being whopped by a woman or locked in the basement is not that bad in the scope of things.
Did you really just argue that an abusive relationship is better than no relationship at all? Really?
Did you also just argue that being locked in a basement is no big deal?
I feel like I'm in some other dimension reading this post.
hm... yeah, I'm a bit confused as well, and wondering who he has in his basement locked up?... nah, jk but still confused how this is not a big deal
OP on the money side, can't he just get a roommate if he wants to split living costs with someone?
Yes, I suppose I did argue that. I don't strive to be politically correct when asked for an honest opinion.
I suppose my advice comes from working in the psych field so long. On the family services side of things it is often the role of the state to keep families together when abuse has occurred, or at least reunite them. This came about because the outcomes are so poor with the foster family system that even an abusive history at home made for better outcomes than foster placement in general.]
So yeah, I do think the OP should have his friend try the Harley method as outlined in "His Needs, Her Needs"
The problem with the "Run Forest Run!" theory is that it doesn't generally work to well in reality. People in abusive relationships will generally end up in new abusive relationships if they leave.
So I'll stick to my guns. From what was suggested by the OP this isn't a situation that can't be changed. But the OP will need to be the one to implement a change strategy.
By the way, I've seen many people who have exhibited some of the negative behaviors noted by the OP that end up becoming very good parents and model spouses.
As at out of the box aside --- The largest an most frequent input into humans is the food they eat. Followed closely by TV. Believe me when I tell you that a diet high in junk food and wheat will have a different behavioral outcome than people eating lots of veggies, healthy meats/fish and healthy oils. Even weird things like red dye can result in behavior issues.
There is also the caffeine and booze effects. You didn't mention if booze was a factor as it often is in these situations. But many people do not realize that caffeine, birth control pills, and antidepressants can have a significant impact on peoples behaviors without anyone realizing this.
Does the GF drink more than one cup coffee per day? Does she take birth control? Is she on other medications? Is your diet generally a wheat based or veggie based diet? What TV shows are you watching? Crime shows tend to legitimize violent behavior while Antiques Roadshow just makes a person nerdier.
I'm not joking about these inputs. What are her inputs? It would be difficult to achieve a different outcome without changing the inputs. Something as simple as getting off birth control may have a huge effect on her personality.
If there is drinking involved, it should be cut drastically or all together.