If someone cuts off communication with you, are they expecting you to be completely apathetic about them to mutual acquaintances?

So there was a girl I used to be friends with in college who cut me off after a misunderstanding. No communication, no chance to explain, flat out refuses to talk with me. We just went from friends to vague hostility. I'd rather not go into what happened, but there was nothing sexual, or inappropriate advances, or anything like that. That happened like a year and a half ago.

Well, it turns out that I'm still friends with her boyfriend on Instagram, and he posted something about her birthday being today on his story. So I responded "Oh, it's [girl]'s birthday today. Hope she's doing well!"

Now, I told my sister about this and she thinks I crossed a boundary, but I frankly don't see the big deal. I guess I just feel like the fact we aren't friends anymore doesn't mean we have to be overtly hostile to each other.

So that got me wondering; what's the expected etiquette around mutual friends when someone blocks you and cuts off contact with you? Are you just supposed to not acknowledge the person's existence?