This is a tough one.. A harrassment warning (In my force anyhow) is exactly as it sounds. A warning!! The normal procedure would be to take a statement from person being harrassed (Your daughter in the presense of an appropriate adult). An officer would go to the offenders house and get them to sign something that states they acknowledge they have been warned and that would be the end of it.
The harrassment warning would sit on file (Not as a criminal record) and if any further incidents occurred that were reported by the aggrieved then a further statement would be taken and the offender would be arrested.
This is all very simple if it is two adults involved but untimately when it's minors then things are more complicated. In short you can't keep your daughter out of it. For a warning to be issued then she would have to provide a written statement (There is no) way round this, and you, as her appropriate adult would be accepting that she would be happy to go to court in relation to the incidents if this continues. If you are not prepared to have your daughter go to court then the warning is toothless. This is something only you (and your daughter) could answer and I have had this conversation with many sets of parents. It certaintly isn't ideal and not something that would normally happen until all other possible courses of action have been looked at.
Harrassment is a funny thing too. Courts look at evidence so make sure your daughter knows not to send any messages the other way that could be deemed similar.
Hope this helps.