True, it's not.
You probably already know this, but pwBPD tend to be hypersensitive. Things that seem minor to a Non can be a life-or-death thing to the pwBPD. Validation is extremely important and a valuable tool to have in your toolbox.
In a case like this, I know you're trying to soothe and calm her. But when you say “This is such a small thing. It's nothing to get upset over” what she may be hearing is “Your feelings are wrong.
You are wrong. I don't care about you or her feelings.” When her daughter doesn't respond, she could be thinking "She hates me. She's ignoring me on purpose. She's dead. I'm never going to hear from her again." Those aren't insignificant things.
Seems irrational, right? But it's true. For pwBPD, feelings = facts. Something that might be a minor sunburn to you is like a third-degree burn needing a complete skin graft to the pwBPD.
We have a lot of tools and articles here that you might want to take a look at:
https://bpdfamily.com/content/listen-with-empathyhttps://bpdfamily.com/content/communication-skills-dont-be-invalidatingGive those articles a look and let us know what you think. Do they make sense? Do you think they're tools you might be able to use?
You may find that if you really listen with empathy and validate her feelings (but don't validate something invalid), you will get a very different response from her and that little breeze may stay a breeze instead of becoming a tornado.
If you were in this situation, what might be a more validating way to respond to her?