I'm kind of torn. I guess I feel like, overall, it's just a cheer--it's brief, ephemeral, and there are surely a lot of them in the world to choose from, so chucking it quietly aside for a few more years doesn't seem like a big deal to me. Especially since the incident was so recent that one of the victims is still on the team--that must be less than five years, right? And, to my mind, how brilliant could this cheer possibly be, that some people aren't willing to give it up--surely we're not talking a Shakespearean epic here or something where you could argue that the "art" trumps the "artist." It would be different, to my mind, if the source of anxiety was a logo the old coach created, which was emblazoned on every uniform, notebook, and even the gym floor, which would be rather more difficult to erase.
But on the other hand, I do see the slippery slope argument. I feel like the cheer could become, or perhaps already has become, something more than a cheer. Obviously, to the victim and other people (like the OP), it's a reminder of a horrible thing that happened not very long ago. But I worry that to some of the older kids who are encouraging its use, it might also become a kind of protest against what happened to the coach, since they consider what he did "no big deal." It could be used in a harassing way against the victims.
I would want to know why the older kids were encouraging its use. If they just like the cheer and think it's cool and don't associate it with anything bad, even though they know who came up with it, surely they could be persuaded that, for the good of the team, they shouldn't use it, because it means something bad to at least one teammate, and there are lots of cool cheers in the world. Maybe their energies could be redirected into coming up with a new, original cheer instead.
As for what the OP should do: If I understand correctly, she's not officially associated with the team or anything like that, right? I think it might carry a certain amount of weight if she approached the head coach as simply a member of the community who attends the games and supports the team, who finds the use of this cheer disturbing for herself (without mentioning the victim at all). She heard them using it and felt sick, and could no longer concentrate on the sport but just kept thinking about what the old coach had done and all the heartache it caused the community. In her opinion, she wishes the head coach would not allow the kids to use the cheer so soon after the incident, if at all. I think this would be perfectly polite and appropriate, and not give away any confidences she might have received. It would be natural to point out the video on a public Facebook page as the way she learned about the cheer's use. Then I would just keep an eye on things and see how the coach handled it.