After thinking about this a bit more my opinion has changed slightly.
Presumably if someone is that on the edge, anything that upsets someone could be the trigger. Must we refrain from saying anything negative to anyone, ever, even if they are bullying or harassing us, just in case they might be on the verge of killing themselves?
That's the thing. If this boy had killed himself because of the girl's response to sexual harassment, he may have took his own life if she had told him to fuck off, or replied in any harsh manner. So if his mental health is that poor anything the OP replied with could have affected him deeply.
Lets keep things in perspective a little.
I would not be impressed if I found my teens typing KYS to anyone, but a 'fuck off you twat' could have also been the straw that broke the camel's back if this boy was in a very bad place.
For some teenaged girls a boy tricking them into viewing porn could also have devastating effects.
What about OP's daughter's mental health? Girls are having to put up with this shit far too frequently and it affects them hugely. Why are some people only concentrating on what her actions could have done to him?
I am not comfortable with the insinuation that OP's DD could have been responsible for a suicide because of the way she reacted to a boy trying to get her to view porn.
Basically, it boils down to this
You’re not allowed a kneejerk reaction, even though you are the aggressed and he’s the aggressor. You carry the responsibility for managing the situation correctly, however much in shock and upset you might be.
Boy sexually harasses girl. Girl responds with KYS and that could have lead to a suicide, she should have thought more about her actions and she deserves to miss out on education for her 'vile' reaction to someone who harassed her. Boy gets less punishment and very little thought is given to how this could have affected the girl.