First you must understand that everything we come into contact with changes us forever. It seems like that factor increases significantly when you increase consciousness to that element, and then include personality, which also further increases that change. Then you must also understand that human beings willingly adapt their own personality to fit more closely in with the personalities of those people who they are drawn to. Add more people, and you get even greater change.
It is likely that even you have experienced this change in yourself, even if you were never aware of it.
Now you must ask, are you willing to accept these changes in this individuals personality, or are you so bothered by them that you can no longer be friends, and are you willing to try to intervene, and would it be wrong to intervene. If these changes are really for the betterment of the individual, then I advise not doing anything to stop it, but if they are corrosive personality trait changes, then a good friend would intervene. Should you choose intervention, I further advise that you only show this person the way, as you only can, and not attempt to force them, as you could generate a violent reaction, as people are only willing to see what they are willing to see, and willing to do what they are willing to do. And should they deny seeing, and deny you, then understand that friends come and go, and you will make more friends, who are more like you are now.
At your age?...Thats going to happen