When she throws litter on the street in front of you don't tell her anything just pick up the litter and throw the litter yourself and I am sure she will feel guilty and with not do same thing again , at least not in front of yoi
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When she does it, go upto her and tell her about what will happen to the society if he does so [like outbreaks and epidemics] if she litters waste like this. Also encourage her to pick it up and put it in the dustbin.
If you want a dialogue:

"Hey, [friend's name]! Don't do that! It's not good for the society! Please pick it up and put it in the dustbin."
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