My friends friend starts a fire and shes 18 but my friend knows and which one gets in trouble?

ok so heres the story two people both girls and well there smokeing a ciggerette and so when the ciggerette is pow one of the girls flicks the stog and then a fire starts so my fren who by the way is 17 and says stop the fire and calls the fire deparment but the other girl says know just leave it. and it wont get anyworse so they leave it and the police shoe up and the fire department stops the fire which one of the girls is the one who gets arrested and there both stilll in school and the 17 year old only has half day shes a senior and the other gurl is 18 and shes also a senior so which one gets introuble for this both or only one of them i say the 18 yea odl should get in trouble and gets all the blame so what do you think