If this was a friend of both you and your uBPDw, and she goes nuclear with the friend, you can only accept that she did that.
If this is somebody that you have an individual friendship with, or somebody you wish to maintain an individual friendship with, you have two issues to deal with:
1. How much you say about your W going nuclear to your friend.
2. (I'm guessing that this happens) Your wife's insistence that you not associate with this (now) horrible person.
Issue #1 is easier than you think. Say as little as possible about BPD or anything specific your wife is going through. Just tell your friend that your wife has her own issues, and those are hers, and you value their friendship, and do not want to end it over your wife's issues.
Issue #2 is simple, but not any easier than you think it will be. All you can do is set boundaries, and refuse to let this happen. The S.E.T. tool can be a helpful one to explain it to her.
A warning: This probably isn't a conscious effort on her part, but isolating you from other influences is part of the system of control that your wife is trying to exert on you. Fighting back against it will require strength on your part--if this is the first time you've defied this sort of effort on her part, she will probably escalate things as you start to resist.
If you haven't read the lessons here, let me recommend these two:
BOUNDARIES: Upholding our values and independenceTOOLS: S.E.T. - Support, Empathy and Truth