Hello bpdwife1000! Welcome to the family!

Your story is not unfamiliar to us here. In fact, it rings a lot of bells for me. I, too, have an undiagnosed BPD husband (or, I suspect he has it -- ultimately, it doesn't really matter since what's important is the behaviors). I know that roller coaster well. For months, I was walking on eggshells, almost afraid to do or say anything for fear it might set him off.
Good for you for going out with your friend! People with BPD often feel threatened by their loved ones' family and friends. This board is full of stories of people who ended up cut off from people who loved and cared about them because they were trying to please their pwBPD. Keep doing the things that are important to you. Keep up the self-care. It's vital that you look after yourself so you can be better equipped to handle your H and take care of your children.
I'm sure you've been giving him unconditional love. But here's the thing with pwBPD: They're like a bucket with a big hole in the bottom. You can try and try to fill it up but it will never be full and, in the process, your own bucket ends up empty and you're exhausted from the effort. That's why self-care is important.
My advice to you when he gets like this? Let him handle his emotions and feelings. Be empathetic and validating but don't cave or coddle.
Another thing is, for pwBPD, what they say they're upset about is sometimes not what they're actually upset about. It helps me to really listen to what my H is saying and try to get at the root of what the REAL problem is. For instance, in a case like this, your going out with your friend could have made him feel threatened and triggered his fear of abandonment (assuming he has that -- most pwBPD do). Does that mean you shouldn't go out with your friend? Absolutely not! But it would give you some clues in how to communicate with him effectively about this.
We have some very good articles that might be useful to you. Have you seen these yet?
https://bpdfamily.com/content/setting-boundarieshttps://bpdfamily.com/content/listen-with-empathyI hope you'll take a look and let us know what you think. And keep posting!