My first suggestion is not to get outwardly anger or frustrated. Feel it but don't initially show. This is a teaching moment.
There are so many ways to respond to this. Either with quotes, personal experiences, songs suggestions, film & book suggestions.
"No one is free until we are all free" -MLK
But in that moment, what do you say that can change that person's mind?
I don't know if there is an answer to this. There is definitely not a universe answer.
I first heard the example of a house on fire from the Macklemore and Ryan Lewis song White Privilege II.
"Black Lives Matter, to use an analogy, is like if... if there was a subdivision and a house was on fire. The fire department wouldn't show up and start putting water on all the houses because all houses matter. They would show up and they would turn their water on the house that was burning because that's the house that needs the help the most."
This example has been used over and over in many different contexts and I find it helpful to understand the true essence of BLM. There have been TikToks about it and comics.
All lives matter yes, but right now black lives are hurting. When someone is hurting, you help them NOW, because they matter NOW. You do not abandon them and say, 'Sorry, I can't help you because everyone is important'. Black lives haven't mattered in America and there are systemic examples (red lining, sharecropping), as well as examples of in-practice (police brutality). Right now we need to step up because the black community (all minority communities) need us to all stand in solidarity NOW!
So ask them, "If you (or your child) broke their leg and was sitting on the side of the road alone, would you want someone to help you? Or would you want them to say, 'sorry all legs matter so I can't help you' and walk away?"
It might make them angry, they might not get it. They might fight back. But that is what they are doing, they are walking away when someone needs them.
You can only educate, discuss, and inform. We can't change people. They need to want to. But that doesn't mean we shouldn't try.