They could just have two separate things happen to them.
For instance (in a movie I saw) two best friends joined different after school clubs (basketball and volleyball). From then, they had different schedules after school and began making friends with their teammates. This started them seeing each other at odd intervals after school ended.
The next year, they then were placed into different classes. One had a friend from her club in the same class and due to them both really enjoying basketball grew really close. Soon it always seemed like the other girl was with them and the conversation seemed to stay on basketball. The volleyball girl felt left out from this. She started doing other things and soon was hanging around other girl who were interested in volleyball like herself.
It was very slow, but over the movie they just ended up not speaking to each other.
To complicate it, one girl could get a boyfriend and other girl could gain some wild friends where she ends up doing some really “stupid” things (drugs, vandalism, assault even).
The girl with the boyfriend could alienate the other due to her constant fixation about him. The other could be rather uninterested in dating/boys so not understand the changes in her friend, but at the same time be jealous of the boyfriend because he is now taking the rest of her friends time that used to be hers. Now she’s rarely seeing her friend at all. Since she’s jealous, she then does not like the boyfriend and they get into arguments over her being rude to the guy. This leaves the other with time on her hands and a suppressed anger. She could make friends who are just as angry and get into fights with people. She could make friends who are into drugs and try some herself. Just some sort of rebellious medium. Whatever it is, she could get caught by some higher authority and then have that to deal with that and its consequences (I’m writing in mind of high school age –> being grounded, suspended from school, juvenile detention even). But at the same time, her friend-with-a-boyfriend could be really against drugs/violence/whatever which then causes more of a rift.
Add to it (potentially):
– the boyfriend could not like the friend back and puts little snide remarks in here and there
– a miscommunication could result in a double stand up (they each thought a different place was the meet location and somehow couldn’t contact each other to reconfirm)
– one friend is rude/mean to a new friend
– one chooses to go to the local college while the other plans to attend some super fancy college the whole content away and never told her
It is like a neat pile of small little things that together make a mountain.