Littering?

I wasn't sure where to post this topic, but I wanted advice, my sixteen year old friend regularly litters and it really annoys me, I find it immature and destructive towards nature, she regularly throws her plastics on the street and doesn't seem to care about it. Personally I do not think think this behaviour is okay, plastic takes years to break down and even at that it never fully decomposes. I don't know what to do, but I hope there is something I can do without it impacting our relationship. What do you think I should do?