It's a tough situation. On the one hand, people could be using the cheer in a totally innocent way, not even knowing the backstory or not connecting it to the cheer, having no clue they're upsetting anyone. Maybe they would be happy to stop using it if someone objected. Or maybe, if someone objected, they would say, "You can't tell us what to do" (kind of a free speech, anti-censorship thing) or "We like the old coach and don't think he did anything wrong, and we want to use his cheer to show support for him." Depending on their response to the objection, very different situations could arise.
But I think someone has to make that objection first, before anything can change. And, as I said before, I think the OP is actually in a good position to do so. She's a community member, but not connected to the team or the school (if I understand correctly). If she were a victim or a family member of a victim, people might (sadly) write her off as just being oversensitive or something. But as a general community member, she could very well represent a broad, general opinion among supporters of the team and the school, who find the use of the cheer tasteless and distracting, especially such a short time after the incident occurred. I think it would be worth conveying her concerns to the head coach from this perspective, and this could be done politely and without overstepping.
If nothing is done, or nothing is done that satisfies the OP, there are further steps she could take, which move the issue more into legal territory.