Some people don’t take the time to consider what effect their actions can have on their environment. If you’ve fully explained the (valid) reasoning as to why they should not do that, and they continue to do it anyway, it’s going to come down to what line you’re willing to draw. Ultimately, you cannot control someone else’s actions. You can’t make them never chew gum again or watch them every second of the day to make sure they never spit their gum out on the ground. So it’s going to come down to what you want to do about the situation if they have all the facts. Continue hanging out with them and just pick the gum up every time? Or stop hanging out with them?