Congratulations to her on her racket and new shoes. Aren't they the most special shoe laces ever -whereas I bet yours, Troisette, are just plain boring- and she should stop acting like velcro
Realistically, and you know this because you are saying you are giving away your power, she is 1 point ahead. If my suspicion is correct, she may have run into you and approached you because she knows you? If this is correct, I also believe that you may have contributed to your demise through your own politeness, respect for social conventions etc as you allowed her all that space automatically treating her as if she is a friend to you. They just invade some space and then expand. If something similar happened, these things are what I would do nowadays. If I'm the point of contact between these people, I do not hesitate taking the leading role (and learn not feel guilty about it).
I know we avoid being rude, we are afraid of appearing insecure etc. But there is a variety of behaviours we can use (but perhaps we somehow have not developed these, they certainly did not come to me automatically). I think I'm an assertive person when it comes to intelligence etc, but very passive in these situations usually.
But here is what I do sometimes and it works:
Say, you run into them on the road. I do not put up a friend face, I put up an acquaintance face, that body language. Imagine she was a talkative and bland landlady. Nothing more than that. I allow them to introduce themselves but take the lead of conversation, if I can't start nodding, move in my space slowly and give body language signals to whomever is with me hoping that they'll understand that it's time to go. I even look at my friend's face. You can always interrupt silently with a friendly look on your face but slightly directed at your friend. He is with you. If he has any interest in you, he should prioritize you. If he doesn't, that's a red flag anyway. This is the early phase of a friendship, he should be on his best behaviour. Then once you get away safely, you can say politely, in a neutral friendly fashion that they are X,Y,Z people who are very good friends with your previous boyfriend. Basically, she is finished

X,Y,Z is not adjectives, it's about their jobs, hobbies etc. The new friend, if he is interested in you and has a healthy mind, will not go near your ex's good friends. If he does, red flag.
If you are sitting somewhere and they come and join, that needs to be stopped with a nod, raising eyebrows. How would you prevent your landlady from sitting with you? Exactly like that. I used to feel guilty. No need. It's not rude. It's just reality. She obviously is not your friend.
At worst, she will tell your ex that you were very cold to her but who cares. You are cold to the guy as well:)) Don't try to be good in their eyes. That's beyond their existential capacity.
If they invite you join, you can decline.
If you are in a circle of friends, move somewhere else to other friends.
This isn't b... .* behaviour. This is polite, indifferent behaviour. She should be thankful for the politeness part.
One thing they may resort to is minimizing us. They somehow try to pass this message: "Oh, darling what have I done to you?" The answer is simply nothing. We should come to the inner point of confidence in ourselves that they cannot get to us. There is a wall of comfortable confidence around us, it's protecting us. Whatever they do, we are not there. We are detached, they are screaming like little children but it's not reaching us. You can even respond with a loving look to these children. Don't forget though. They are not your children. If they increase the attack, give them a blank or surprised, inquisitive look - "Oh, sorry, did I do something wrong to you?" This turns the tables down.

Personally, I think I will always feel insecure or uncomfortable instinctively around these women. No matter how much I heal, this may stay with me. This isn't about the boyfriend. This is about me. Owning and accepting it helps me - at least I now have cognitive power around this. I feel they have something I don't have. They seem very relaxed, I'm more stiff with people until I get more comfortable. They flirt with their bodies, I flirt with my intelligence. Men seem to be relaxed around them, I sometimes scare men. Well, insecure men. I'm not looking for the type of man that finds that attractive. It's a good test. Also, I try to remember my advantages as a person. If I feel jealous, there is a rule: If there is one thing, one thing about me that I like and think she doesn't have, jealousy is not allowed. I apply this like a rule. I remember my good things and relax in my own body. You can practice this when you are tense at home. Learn to remember good things about yourself and relax in your own body.
I'm of the opinion that we are the real targets of these narc women. It's a silent competition between females and men like being so unaware of it always:)) I wouldn't be the one bringing this up until I'm very close with a man. I think these women enjoy competing with us more than they actually enjoy company of our friends:)) She probably feels very insecure near you but you can't see this because you are worried about her power over you. That power is imaginary. It's in your imagination and perhaps a conditioning of previous triangles, too. (Well, the men who created them were insecure and unworthy.)
A narc can only get through to you if you have holes in your shield. Your insecurities can become your power. They are telling you something about yourself. If you can accept those things and stop seeing them as weaknesses they cannot do anything. It's just difference. Difference is good. We all have different things to offer to the world. We should be adamant and just accept people who really really really appreciate who we are. Let the rest talk about shoelaces and racquets. The question should not be "Will I lose my friend to her?" It should be "how much would I really appreciate someone who falls for this woman?"
If there really is competition, if you wan't to join in, I wouldn't compete when she chooses to or when she is around. I would genuinely try to strengthen my own bonds with the person.
This is my polite "back-off" strategy with narcs. Other things are possible:))
Also, please do not carry your NC like a secret. Disclosure is stressful sometimes. You don't need to disclose anything to anyone before you are ready. But there is nothing with not wanting to socialize with your ex's good friends (actually, supplies and confidantes/confidants). Do not get into their triangles. I would drop the hint about who they are to my friend and then let all of them socialize as much as they like. I'd just carry on with my own relationship with the friends, and soon you'll see who is on whose side etc very clearly anyway. If it comes to worst, your ex and this friend will be having dinner at this couple's house and how attractive is that to you anyway. Bon appetite to them:))