First, welcome to SI. I'm sorry you have to go through this. I, too, think that everything you've described about what's going on inside you is normal. Being betrayed is awful, and it's traumatic to your self, and focusing on your healing - healing, not R, not D - is the quickest way back to a joyful life. And you most definitely can heal, survive, and thrive.
Like BFTG, I urge you to reframe your thinking about boundaries. You can't control her, not because she's irredeemable but because one person can't really control another person except through force, the threat of force, or the fear of force.
Boundaries are for you. They are very close to ultimatums - if someone does X, you'll do Y. If your W continues with this friend, you'll ????. But you have to be ready to impose the consequence you threaten. If your W doesn't dump her adultery co-conspirator (I hope the originator of that term agrees it applies here), you'll either have to fill in the '????' and act accordingly or lose credibility with your W.
You'll be OK either way your W goes. You'll be OK if she dumps her friend. You'll be OK if she doesn't. You have to risk your M to save it.
A good R will require a lot more than dumping her friend. It's best if you decide on your own requirements, but here are some things that have helped make some good Rs.
Your W doesn't look like a good candidate for R at this point, and she doesn't even look like she's likely to become a good candidate. Yours is a case in which immediate D may be your best bet, based on what you've written. D right now is certainly a good choice, but you don't need to act right this minute. If R holds some attractions for you, keep reading. Your W would not be the only serial cheater who redeemed herself, and she wouldn't be the only lousy candidate for R who became a good one.
Let's go back to basics:
I recommend thinking of R as 3 healings:
1) You heal you. Most BSes are inundated with immense amounts of one or more of grief, anger, fear, shame on d-day. The largest part of your work is to process those feelings out of your body. A good IC can help you do this.
2) Your WS heals themmself. They need to change from cheater to good partner. I think that requires IC for the WS, but others disagree.
3) Together you build a new M.
This means you can recover from being betrayed without your WS; that is, you can survive this crisis and thrive without your WS, but you need your WS to R. You can heal yourself because you control yourself. You don't control your WS. so you can't control her healing or R. I recommend making survive and thrive your primary goal and R your stretch goal.
Have you read the Healing Library here? If not, there's a lot of good stuff there. Click the link in the yellow box in the upper left of the SI pages.
I think there are a number of keys ingredients to R.
First, what do you want? Do you really want R? If not, don't lie to yourself - D is a moral response to being betrayed. R is hard work, and wanting it makes it less difficult. I'd go so far as to say that it's impossible to achieve unless you want it.
I recommend figuring out your requirements for R and seeing if your W will sign on. If they won't, perhaps they can come up with something else that will meet your requirements, but if you can't negotiate something truly acceptable to both of you, great - you can go directly to D. Otherwise, you can monitor them for 3-6 months and commit to R for yourself if they are (is?) consistent in meeting your requirements.
The requirements need to be observable and measurable. That way it's easy to monitor progress and make
adjustments as you go along.
Common requirements include:
NC - no contact with ap; if ap initiates contact, report to BS and together decide how to respond
Transparency - BS has passwords to e-mail, voice-mail, phones, etc.; WS keeps BS informed of whereabouts,
activities, and companions at virtually all times
Honesty - WS answers BS's questions when they're asked, although sometimes a break is necessary, sometimes an answer is best deferred to MC session, etc., no more lies.
IC for WS - to change the thoughts and feelings that supported the A, with signed release that enables C to talk with BS about WS's goals and progress (so the BS can make sure WS's IC isn't being lied to).
IC for BS - for support - and for resolving any internal issue that comes up
MC - to help communications between the partners
Some (Most?) people have individual requirements - my W had to arrange dates for us on a weekly basis and must initiate sex sometimes. What do you want from your W?
And R is a joint endeavor - if one of you hides objections to the other's requirements, you sabotage R. And you have to see your WS as a human being of worth equal to your own to make R work. You don't have to see your WS as a human being whose worth is equal to your own, but you sure can't R, except with an equal.