You either have to read your son the riot act forbidding him to open his mouth, or you need to respect your daughter's wishes and let it be her day. I know it's hard when our kids don't get along, but sometimes we just have to try not to "fix" things, particularly since it probably can't be solved between now and graduation. Maybe this will be a wake-up call to your son that he's being a jerk and needs to knock it off.